[chore] media and emoji refactoring (#3000)

* 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

internal/workers/workers.go

@@ -49,10 +49,6 @@ type Workers struct {
 	// for asynchronous dereferencer jobs.
 	Dereference FnWorkerPool
 
-	// Media provides a worker pool for
-	// asynchronous media processing jobs.
-	Media FnWorkerPool
-
 	// prevent pass-by-value.
 	_ nocopy
 }
@@ -84,10 +80,6 @@ func (w *Workers) Start() {
 	n = 4 * maxprocs
 	w.Dereference.Start(n)
 	log.Infof(nil, "started %d dereference workers", n)
-
-	n = 8 * maxprocs
-	w.Media.Start(n)
-	log.Infof(nil, "started %d media workers", n)
 }
 
 // Stop will stop all of the contained worker pools (and global scheduler).
@@ -105,9 +97,6 @@ func (w *Workers) Stop() {
 
 	w.Dereference.Stop()
 	log.Info(nil, "stopped dereference workers")
-
-	w.Media.Stop()
-	log.Info(nil, "stopped media workers")
 }
 
 // nocopy when embedded will signal linter to