Documentation for Issue Boards

## What does this MR do?

Add documentation for issue boards.

## What are the relevant issue numbers?

- Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/17907
- Backend MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5548
- Frontend MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5554

See merge request !5713

Change slash commands docs location



See merge request !5937

Fix typo in gitlab-workhorse header

See merge request !5891

Document protected branches new behavior



See merge request !5934

Fix empty dropdown

Deselecting the current selected option on a dropdown they were removing the input related to them.
This led to a unexpected cases. The forms were submitted without all required inputs and the label was still indicating an option was selected. This MR fixes that.

See merge request !5927

Before this the dropdowns were not being disabled if the user unselected an option from the allowed to merge/push dropdowns.

No need to update the branch permission if no option is marked on the dropdown

This makes dropdown to look according the dropdowns on the edit form. This options add a tick mark next to the selected option.

Fix inputs not being found due to incorrect class in wrap element

Fixes failing spec

See merge request !5926

Fix Error 500 resulting when loading network graph

`discussion_id` may not be present when the SELECT call for notes does not include this attribute. Don't attempt to set the discussion ID unless the model contains the attribute:

```ruby
irb(main):019:0> notes[0]
  Note Load (10.3ms)  SELECT notes.commit_id, count(notes.id) as note_count FROM "notes" WHERE "notes"."project_id" = $1 AND (noteable_type = 'Commit') GROUP BY notes.commit_id  [["project_id", 13083]]
ActiveModel::MissingAttributeError: missing attribute: discussion_id
```

Closes #21119, #21128

See merge request !5922

`discussion_id` may not be present when the SELECT call for notes
does not include this attribute. Don't attempt to set the discussion ID
unless the model contains the attribute.

Closes #21119, #21128

Fix assorted rspec failures due to stale, cached user permissions

RequestStore is disabled in tests, but the Ability class was caching user permissions based on the user and project ID of previous test runs. Revise code to use RequestStore only if it is active.

See merge request !5919

RequestStore is disabled in tests, but the Ability class was
caching user permissions based on the user and project ID of
previous test runs. Revise code to use RequestStore only if it
is active.

Update gitlab-shell to v3.4.0

Part of Redis Sentinel support: gitlab-org/gitlab-shell#29

See merge request !5920

Backport  changes from gitlab-org/gitlab-ee!581



See merge request !5884

Disable “issue by email” feature until it uses a different token

Separate token being worked on in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5914, but too late for 8.11.

See merge request !5915