Load milestone tabs asynchronously
What does this MR do?
Loads all the milestone tabs asynchronously to increase performance.
The tabs loads async are:
- Merge requests
- Participants
- Labels
These are only loaded when the user specifically clicks on that tab.
Screenshots (if relevant)
What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ce/issues/27387#note_27961225