non-fixed top navigation hides alert banners when scrolled down (call for new alert notification UI system)

Problem

With the change to a non-fixed top navigation, error and alert banners are no longer immediately visible anymore when for example you are scrolled down in the conversation in an issue or merge request thread. So in a way this can be considered a regression

Proposal

This calls for a change in how our alert/error system should look and function. I think these messages have a high priority to be read as soon as someone opens the view, be it a newly loaded view or already existing. This especially becomes true with all the real-timeness we are introducing. A user should be able to know if a page is still loading new content yes or no.

Therefore this information should function/show up regardless of scroll position and thus should require its own sticky ui element on the page.

Inbox by Google has imo a great implementation of notifying the user of problems, general information and let them act upon it:

CC/Call to actions

@pedroms (we discussed this in chat), @victorwu (this is related to the issue on real time issue descriptions i believe), @sarrahvesselov @cperessini @tauriedavis