Prevent flash alert text from being obscured when container is fluid

What does this MR do?

Fixes a regression that causes the text within server-initiated flash alerts (not flash notices) to be obscured when .container-fluid is applied. It was reported in relation to a cherry-picking flash alert, but the problem occurs with all server-initiated flash alerts contained by a fluid container.

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

Note that as part of this fix, I combined some duplicate code and removed .flash-text & .content-wrapper selectors as they didn't seem to be necessary.

I manually tested JS-initiated Flash alerts to make sure this doesn't affect their appearance. I also grepped around for anything else that might depend on these styles. Everything checked out. But... can you see something this is going to break?

Why was this MR needed?

Affected users can't read flash alerts.

Screenshots (if relevant)

Before:

After:

JS-initiated flash alert (unaffected):

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 merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/22915

cc: @jschatz1