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Extract parts of LegacyDiffNote into DiffOnNote concern and move part of responsibility to other classes

This only supports Issues and MergeRequests right now because of the
consistency of the routes those models provide.

This reverts commit 13e37a3e.

Allow users to request access to projects and groups

See merge request gitlab-org/gitlab-ce!3798
Signed-off-by: Rémy Coutable <remy@rymai.me>

+ Move 'Edit Project/Group' out of membership-related partial
+ Show the access request buttons only to logged-in users
+ Put the request access buttons out of in a more visible button
+ Improve the copy in the #remove_member_message helper
Signed-off-by: Rémy Coutable <remy@rymai.me>

Closes #18484

- Added endpoint to retrieve approval status of MR
- Added endpoint to approve MR

Add a new application setting, after_sign_up_text. This is text to be
rendered as Markdown and shown on the 'almost there' page after a user
signs up, but before they've confirmed their account.

Tweak the styles for that page so that centered lists look reasonable.

- To hold registrations from U2F devices, and to authenticate them.
- Previously, `User#two_factor_enabled` was aliased to the
  `otp_required_for_login` column on `users`.
- This commit changes things a bit:
    - `User#two_factor_enabled` is not a method anymore
    - `User#two_factor_enabled?` checks both the
      `otp_required_for_login` column, as well as `U2fRegistration`s
    - Change all instances of `User#two_factor_enabled` to
      `User#two_factor_enabled?`
- Add the `u2f` gem, and implement registration/authentication at the
  model level.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Closes #15638

Signed-off-by: Rémy Coutable <remy@rymai.me>