Check checkbox remove source branch

What does this MR do?

Mainly a couple of things:

1. Makes sure you cannot remove a branch if it has another MR which is still open with the same source_branch (2773d3ff)
2. Create a remove_source_branch field on the merge_requests table so we don't need the keys (should|force)_remove_source_branch from the merge_params anymore

• as the default is set to true, when creating a new MR this checkbox will default to true

1. Update the styling of the MR docs -- tried to be a good boyscout

Are there points in the code the reviewer needs to double check?

The name force_remove_source_branch seems terrible as it didn't force anything, I checked, and checked again, but this seems really the case. Why should_remove_source_branch wasn't used in the first place is a mystery to me too.

Why was this MR needed?

Storing YAML in a text field, with two different keys seems like a terrible idea. Also, the checkbox should be checked by default.

What are the relevant issue numbers?

Fixes #14413 (closed)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together
• Test migration on MySQL