Cache REST data for Vue MR list filters
Some filters can use an autocomplete cache (e.g. My Reaction) because they have a finite list of options (emoji), while other filters have arbitrary-length lists (e.g. branches).
These arbitrary-length lists need to be cached at least at the component level - but ideally at the network level - so that subsequent requests for (as an example) the branches don't each result in the same network request to fetch the list of branches that is unlikely to have changed substantially in the course of a single page load.
The non-Vue MR list page seems to already have this feature, which implies it's available either as a function of the Rails render (won't be possible to use directly), or as a feature of the search bar as implemented by the HAML page (should be possible to use in an identical way).
This issue is partially investigation ("how is it working today on the HAML page?") and partially implementation ("make it also work on the Vue page.").