Move global flash messages to page content

The idea of moving flash messages to the top of each page came up at different places. Now discussion is scattered around several issues and hard to follow. Let's make it one place.

Related issues: #19290 (closed), #19291 (closed), #19378 (closed)

Current state of discussion:

@winniehell https://gitlab.com/gitlab-org/gitlab-ce/issues/19290#note_12833946

@dzaporozhets https://gitlab.com/gitlab-org/gitlab-ce/issues/19378#note_12831748

@winniehell yes lets do it

@skyruler https://gitlab.com/gitlab-org/gitlab-ce/issues/19291#note_12943869

it shouldn't be above the header cause it loosing the context, actually it should be something like on @dzaporozhets mockup

@tauriedavis https://gitlab.com/gitlab-org/gitlab-ce/issues/19378#note_12959447

Moving the broadcast to the top makes sense to me. I increased the height slightly to align with the menu collapse and added a close button like mentioned in the issue for flash messages.

cc @dzaporozhets @jschatz1 @skyruler @tauriedavis @cperessini — I hope I got everybody