Prevent flash alert text from being obscured when container is fluid

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.

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?

Affected users can't read flash alerts.

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

See merge request !6694
Signed-off-by: Rémy Coutable <remy@rymai.me>