Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4964#note_12741046

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4093#note_12571602
and
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4093#note_12571670

This reverts commit 13e37a3e.

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

This is done to make belongs_to with required to properly validate association.
Otherwise `ActiveRecord::StatementInvalid` is raised.

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

The schema doesn’t reflect the changes of the last 3 migrations:

* 20160610140403_remove_notification_setting_not_null_constraints.rb
* 20160610201627_migrate_users_notification_level.rb
* 20160610301627_remove_notification_level_from_users.rb

Using update_column to store the boolean flag to avoid
any side effects with the current state of the project
instance

The logic of the method was obviously inverted.
Signed-off-by: NRémy Coutable <remy@rymai.me>

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

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.

This improves performance of the duplicate notification settings
migration by removing duplicates in batches instead of using one big
"DELETE FROM" query.

The previous query would locally run over 45 minutes without even
finishing. This new setup finished in a matter of seconds.

Fixes #18289

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

- 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 #17920