Avoid conflicts when merge CE to EE by backporting code from EE.

Instead of checking in `SlashCommands::InterpretService` what the issuable the
type of the issuable is, ask the issuable if it is capable to do those thing and
implement it in the issuable itself. The issuable will check if it's possible
and if the licensed feature is available.

This should also make it easier to ever add multiple assignees to MergeRequests.

The list of attributes for the filtered search input was getting long, so use a
helper method to fill that hash.

Also, for multiple issue assignees, a helper is more convenient because it would
allow EE to override the behavior if MIA is supported.

Avoid code duplication and limit the number of CE -> EE merge conflict by
reusing `FormHelper#issue_assignees_dropdown_options` to set some assignee
dropdown attributes.

To make the code back in line with EE.

[ci skip]

Only EE supports multiple issue assignees, so this CE code should not contain
code to have multiple assignees. EE will override the multiple issue assignees
feature by overriding this method.

Remove backticks from chat messages because they're not rendered as code

See merge request !12276

Stop showing task status if no tasks are present

Closes #33880

See merge request !12303

Previously the task status would be visible even if there where no tasks
in the description. This fixes that by changing the regex to check for
tasks without 0.

This also fixes a bug where update the description inline would not
update the task status text on screen.

Closes #33880

Clean up Vue warnings in tests

Closes #33624, #32965, and #33622

See merge request !12163

Merge branch '33734-transient-karma-failure-on-master-for-date-time-utils-timefor-returns-remaining-time-when-in-the-future' into 'master'

Resolve "Transient karma failure on master for `Date time utils timeFor returns remaining time when in the future`"

Closes #33734

See merge request !12227

Resolve "Transient karma failure on master for `Date time utils timeFor returns remaining time when in the future`"

Fix hover style for emoji button on parallel diff view

Closes #33865

See merge request !12281

Fix note highlight being added when you don't have a URL fragment hash #note_xxx

Closes #33916

See merge request !12293

Fix issue description syntax highlighting and math rendering

Closes #33877

See merge request !12282

Update rainbow from 2.1.0 to 2.2.2.

See merge request !12289

Update paranoia from 2.2 to 2.3.1.

See merge request !12288

Remove implicit dependency on `gitlab_sign_in` assigning to a user instance variable

See merge request !12296

Update carrierwave from 1.0.0 to 1.1.0.

See merge request !12291

We shouldn't be using instance variables in specs to begin with, and
depending on this implicit behavior of `gitlab_sign_in` would have made
it more difficult to change to `sign_in` where possible.

...we've also gone ahead and changed to `sign_in` where possible.

Prepare for bypassing GitLab sign in in feature specs

See merge request !11913

Not sure why this fails with the full login flow, but it's not necessary
and would've been changed eventually anyway, so whatever!

Change `logout_direct` uses to `gitlab_sign_out_direct`

Change single `login_via` use to `gitlab_sign_in_via`

Always use a explicit true/false value to set highlight because jQuery
assumes `null` means toggle from whatever state and doesn't force false.

Regressed in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12098

Restored missing function inside dropzone_input.js

Closes #33836

See merge request !12279

Enable autocomplete on project snippets

Closes #33911

See merge request !12286

- [x] Rename `login_as` to `gitlab_sign_in`
- [x] Rename `login_with` to `gitlab_sign_in_with`
- [x] Rename `login_via` to `gitlab_sign_in_via`
- [x] Rename `logout` to `gitlab_sign_out`
- [x] Rename `logout_direct` to `gitlab_sign_out_direct`
- [x] Make `gitlab_login_with` (née `login_with`) private
- [x] Make `mock_auth_hash` private

Changelog: https://github.com/carrierwaveuploader/carrierwave/blob/f2f8569b1013f601fe7be4b3a616359acd27240d/CHANGELOG.md#110---2017-04-30

Adds support for Rails 5.1

Changelog: https://github.com/sickill/rainbow/blob/master/Changelog.md#222-2017-04-21

Adds support for Ruby 2.4.

https://github.com/rubysherpas/paranoia/blob/core/CHANGELOG.md#230-2017-04-14

Adds support for Rails 5.1.

Adjust position and wording for related issues in merge requests

Closes #33652

See merge request !11759

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1210 disabled
autocomplete for personal snippets and project snippets. But we want
autocomplete on project snippets.

Re-enable autocomplete for milestones, tags, releases, and wiki

Closes #33598

See merge request !12110

Disable autocomplete on snippets comments

Closes #33594

See merge request !12108