Reduce number of loading spinners in issuables sidebar

Problem

When the issuable sidebar is loading (in issues, epics, MRs), there are too many and inconsistently placed loading spinners:

CleanShot_20210513111843

Proposal

Instead, consider:

1. Immediately showing all of the fields with their titles.
2. Hide the “Edit” buttons until the corresponding field is loaded.
3. While loading, use skeleton loaders to represent the final content.