Ensure `runners_token` in project

See: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2294 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2294#note_3135776

The problem is that runners_token is not properly ensured. This commit https://gitlab.com/gitlab-org/gitlab-ce/commit/8cdd54cc0696b76daa2baf463d02d944b50bac6a introduced runners token in a way, that doesn't work for previous installations, where runners_token is nil.

Recently, I fixed similar problem with runners_registration_token, see here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2039 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2185.

My approach was to refactor TokenAuthenticatable to make it reusable. Then it is possible to add to model:

include TokenAuthenticatable add_authentication_token_field :runners_token

> This adds several methods to `Project` -- method that generates token, and several other methods. With this approach then we can add this method to `Project`:

> ```ruby
def runners_token
  ensure_runners_token!
end

If someone checks runners token it will be returned if it exists. If it doesn't exist -- it will be generated instantly, saved to database and returned.

As a matter of fact, currently, users can prevent errors by editing project, as runners_token will be generated on before_validation, according to https://gitlab.com/gitlab-org/gitlab-ce/commit/8cdd54cc0696b76daa2baf463d02d944b50bac6a.

\cc @stanhu

mentioned in merge request !2294 (merged)

Status changed to closed

Fixed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2294.

mentioned in issue #13272 (closed)