Check if we can unify collapsible patterns across the product

I discussed this with @danmh the other day as we have quite a few different collapsible patterns floating around in the product. This issue aims to collect all different collapsible patterns used in the product and to see which ones can be unified.

Some examples

Threads:

Diff threads:

Settings block:

MR Widget:

Issuable widget (Crud):

MR request overview:

Inline collapsible:

So there might be some potential to unify at least some of them.