Improve UX for multiple assignees in merge requests

• Backend weight: 1
• Frontend weight: 3

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As a follow-up of https://gitlab.com/gitlab-org/gitlab-ee/issues/2004, this issue will further improve the UX for multiple assignees in merge requests, as in the %11.10 we've simplified to just changing the tooltip messages of the "Cannot merge" alert icon.

From https://gitlab.com/gitlab-org/gitlab-ee/issues/2004:

• Change the "Cannot merge" icon (red is to heavy)
• Change how this is displayed slightly with multiple people
• Ordering the current assignees so that people that can merge appear first (to improve scan ability)
• The tooltip texts should be:
  • If all assignees cannot merge: No one can merge
  • If the current assignee cannot merge: Cannot merge
  • If some assignees cannot merge: 2/5 can merge

svg:

• color: #FC9403
• warning_exclamation_mark.svg

Backend gotchas

• The auto-complete endpoint currently doesn't return a can_merge data for each user, that's because it doesn't have the context of the merge request in its params. Backend will be required here.

cc @pedroms @DouweM