Geo: Small documentation changes and fixes

See merge request !1158

Fixed merge request environment not displaying

Closes gitlab-ce#27080

See merge request !1154

Use delete_all in the gitlab:elastic:clear_index_status rake task

See merge request !1143

Pull EE specific Gitlab::Auth code in to its own module

See merge request !1112

Git operations cleanup

See merge request !1151

* ce/fix-git-hooks-when-creating-file: (78 commits)
  Make sure different project gets a merge request
  Try to check if branch diverged explicitly
  We don't care about the return value now
  Make GitHooksService#execute return block value
  Use commit rather than branch, and rename to avoid confusion
  @tree_edit_project is no longer used
  Fix renaming
  Rename from base to start because base could mean merge base
  Detect if we really want a new merge request properly
  Properly fix the edge case!
  Rename source to base to avoid confusion from MR
  Don't set invalid @mr_source_branch when create_merge_request?
  Prefer leading dots over trailing dots
  Fix for initial commit and remove unneeded args
  Just trust set_commit_variables to set everything!
  Prefer leading dots over trailing dots
  Merge request terms are reversed for GitOperationService
  I think I am really confused, should be @tree_edit_project
  Remove tag with git hooks
  Introduce Repository#with_repo_branch_commit
  ...

Add username and password to Jenkins hook url

Closes #804

See merge request !1035

Allow squashing MRs

Closes #150

See merge request !1024

Update guidance on handling shared secrets in HA setup

See merge request !1152

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/27080

Snippet spam EE

See merge request !1148

Only a tiny portion of code in EE for Gitlab::Auth was different
from CE. This had the potential to result in merge conflicts.
Now, it's moved in to its own EE module and is prepended into the
Gitlab::Auth class.

Apply the same spam checks to public snippets (either personal snippets
that are public, or public snippets on public projects) as to issues on
public projects.

When a snippet is submitted, but there's an error, we didn't keep the
visibility level. As the default is private, this means that submitting
a public snippet that failed would then fall back to being a private
snippet.

Resolve ""Merge when pipeline succeeds" visible if approvals are pending"

Closes #1595

See merge request !1101

Add info on using both push/pull repository mirroring

See merge request !1146

[ci skip]

This avoids the need to:

1. Perform a full clone.
2. Deal with protected branches (because there is no push).
3. Deal with repository push events (because there is no push).

1. When a branch is protected by a wildcard, it can't be removed by
   anyone. Skip hooks on branch removal after squashing so that the
   branch is always removed.
2. Pushing a branch creates an event, cluttering the activity
   stream. Manually remove this, too.

If the temporary branch we want to push to is protected by a wildcard,
we need to add a temporary protected branch entry to allow the current
user to push to it, even if they can't administer the project.

In this case, we can save a lot of time and gymnastics by just
immediately returning the commit SHA. If it can't be merged, the
squashed commit couldn't be merged either.

* 'Learn more' should be 'about this feature'
* The feature is to squash and merge at once, not squash before merge