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
fix high cpu usage issue that occured in Firefox

fixes issue related with #342 (Merge Request view causes 100% CPU in firefox)
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
Gitlab::Git::Compare in Satellite::CompareAction
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
Do not execute hooks on empty projects, fixes #6108
7.2 started
Improve markup gems
Add omnibus-gitlab to the monthly release process
Only redirect to referrer from public GitLab pages
Log Sidekiq arguments