Make selected labels visible while labels form is being used

Problem to solve

When opening a Merge Request, the author isn't able to see which labels they've already selected in the UI.

Here I've selected three labels, but the only way to find out which three (other than "Verify") is to scroll or filter through the entire dropdown until I can find a checkmark.

Intended users

Anyone opening an MR and needing to tag it properly for tracking. Engineering is currently improving and codifying our use of certain labels, so it's important for an MR author to be able to see what they've applied and make sure they don't miss anything.

Proposal

@gtsiolis mentioned proposals from both #34262 (closed) and #28804 (moved), and @mvanremmerden also brought up making them removable from a top-section like this:

This might also be an opportunity to unify this design application-wide.

What does success look like, and how can we measure that?

Success would be making sure that all currently applied labels are visible while the UI for adding labels is open and being used. This should be tested with a large number of labels, as Issues and MRs alike both can have quite a lot.

Links / references

Original Slack thread: https://gitlab.slack.com/archives/CETG54GQ0/p1561149801177800