Prevent flash alert text from being obscured when container is fluid

## What does this MR do?

Fixes [a regression](https://gitlab.com/gitlab-org/gitlab-ce/commit/a2af7790) 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:
![Screen_Shot_2016-10-05_at_3.02.38_PM](/uploads/af258585e428d82dc2b293434661790b/Screen_Shot_2016-10-05_at_3.02.38_PM.png)

After:
![Screen_Shot_2016-10-05_at_3.02.47_PM](/uploads/75a93235440e2b83088e66bbe53d6cee/Screen_Shot_2016-10-05_at_3.02.47_PM.png)

JS-initiated flash alert (unaffected):
![Screen_Shot_2016-10-05_at_3.39.57_PM](/uploads/ba678cfaeb09c639499059712a33abe8/Screen_Shot_2016-10-05_at_3.39.57_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

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

cc: @jschatz1 

See merge request !6694