* start updating media manager interface ready for storing attachments / emoji right away
* store emoji and media as uncached immediately, then (re-)cache on Processing{}.Load()
* remove now unused media workers
* fix tests and issues
* fix another test!
* fix emoji activitypub uri setting behaviour, fix remainder of test compilation issues
* fix more tests
* fix (most of) remaining tests, add debouncing to repeatedly failing media / emojis
* whoops, rebase issue
* remove kim's whacky experiments
* do some reshuffling, ensure emoji uri gets set
* ensure marked as not cached on cleanup
* tweaks to media / emoji processing to handle context canceled better
* ensure newly fetched emojis actually get set in returned slice
* use different varnames to be a bit more obvious
* move emoji refresh rate limiting to dereferencer
* add exported dereferencer functions for remote media, use these for recaching in processor
* add check for nil attachment in updateAttachment()
* remove unused emoji and media fields + columns
* see previous commit
* fix old migrations expecting image_updated_at to exists (from copies of old models)
* remove freshness checking code (seems to be broken...)
* fix error arg causing nil ptr exception
* finish documentating functions with comments, slight tweaks to media / emoji deref error logic
* remove some extra unneeded boolean checking
* finish writing documentation (code comments) for exported media manager methods
* undo changes to migration snapshot gtsmodels, updated failing migration to have its own snapshot
* move doesColumnExist() to util.go in migrations package
