Import the database as the `git` user

This ensures that all tables created during the import belong to `git`.
If you import as a different user, such as the `postgres` superuser, you
may encounter issues where the GitLab database user cannot access tables
in gitlabhq_production, _even if_ `git` is the owner of
gitlabhq_production at the time of import.
Update markdown reference to external issues

1. Issue may be not only in jira.
2. Rewrite method for support different external issue trackers
prevent events loading while loading events refs #49759
Update 6.9-to-7.0.md

fix gitlab.yml example diff command
Delete mailer queue because we don't use sidekiq_mailer gem and now the mailer queue doesn't exist any more.
change bash to sh
fix bash-ism
Fix milestones description's max length
Fix 404 error when clicking the branch link on hipchat message
Add full commit message to commit api as field message.
Include "Merge-request reopened" emails in the proper email thread
Fix missing links and incorrect display of issue numbers for notes, comments, and merge requests
Add an option to GET /projects in the GitLab API to exclude archived projects
Less verbose HipChat integration

Whenever a push with a lot of merges is made, the HipChat channel gets
overwhelmed. It is easy to loose everything else in all the GitLab
messages. It makes more sense to limit the number of commits shown.

The same problem happens with longer commit messages (like this one). It
will be useful to show only the subject of the commit message, not the
full one.
Prevent 500 if hook url is down, fixes #7376
Dont stub gitlab-shell calls. Make test environment closer to real one
Fix 500 error when rename repository. Fixes #1473
Uncomment change branch test
Fix tags spinach test
Fix browse code spinach tests

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>