Eager load milestone and labels when creating issues and MRs

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Description

On the issue and MR creation pages, the milestone and label dropdowns should start loading their contents in the background before the user opens them, but only if the user can actually set them. Often on GitLab.com, when I am creating an issue/MR, it takes a long time for the milestones and label dropdowns to load. It is often faster for me to submit the form and edit the issue after it is created.

Proposal

  1. When the issue/MR creation page loads, start AJAX requests to get the available milestones and labels.
  2. If the user opens either of the dropdowns, use the pending (or completed) AJAX request instead of starting a new request (and therefore making the requests from step 1 pointless).

Suggested labels

gitlab-ce2656755 gitlab-ce2655208 gitlab-ce2655237 gitlab-ce2658854 ~Frontend ~Performance

Edited by 🤖 GitLab Bot 🤖