Simplify database replication instructions and make dryer
What does this MR do?
Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/4254 and closes https://gitlab.com/gitlab-org/gitlab-ee/issues/4265 and attempts to make database_source.md much DRYER, while also clarifying database.md.
Are there points in the code the reviewer needs to double check?
I mostly left the section with the replication script "as is" because work that @nick.thomas is doing in omnibus-gitlab#2925 (closed)
Why was this MR needed?
For more clarity and less duplication.
Screenshots (if relevant)
Does this MR meet the acceptance criteria?
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Changelog entry added, if necessary -
Documentation created/updated -
API support added -
Tests added for this feature/bug - Review
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Has been reviewed by UX -
Has been reviewed by Frontend -
Has been reviewed by Backend -
Has been reviewed by Database
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Conform by the merge request performance guides -
Conform by the style guides -
Squashed related commits together -
Internationalization required/considered -
If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan