Merge branch 'flash-cherry-pick-error' into 'master'

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

CHANGELOG

@@ -33,6 +33,7 @@ v 8.13.0 (unreleased)
   - Only update issuable labels if they have been changed
   - Take filters in account in issuable counters. !6496
   - Use custom Ruby images to test builds (registry.dev.gitlab.org/gitlab/gitlab-build-images:*)
+  - Prevent flash alert text from being obscured when container is fluid
   - Append issue template to existing description !6149 (Joseph Frazier)
   - Trending projects now only show public projects and the list of projects is cached for a day
   - Revoke button in Applications Settings underlines on hover.

app/assets/stylesheets/framework/flash.scss

@@ -21,7 +21,8 @@
   .flash-notice, .flash-alert {
     border-radius: $border-radius-default;
 
-    .container-fluid.container-limited.flash-text {
+    .container-fluid,
+    .container-fluid.container-limited {
       background: transparent;
     }
   }
@@ -35,12 +36,6 @@
   }
 }
 
-.content-wrapper {
-  .flash-notice .container-fluid {
-    background-color: transparent;
-  }
-}
-
 @media (max-width: $screen-md-min) {
   ul.notes {
     .flash-container.timeline-content {