Prevent web-server to start without a working db connection"

This MR adds logic where web_server (unicorn or puma) will refuse to start if ActiveRecord connection is not established (better than starting in a broken state that can only be resolved by a restart). In the omnibus case, runit will bring unicorn back regularly, and once the database problem is solved, the site will recover automatically.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #7003 (closed)