[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/processing/admin/email.go

@@ -55,7 +55,7 @@ func (p *Processor) EmailTest(
 		InstanceName:    instance.Title,
 	}
 
-	if err := p.emailSender.SendTestEmail(toAddress, testData); err != nil {
+	if err := p.email.SendTestEmail(toAddress, testData); err != nil {
 		if gtserror.IsSMTP(err) {
 			// An error occurred during the SMTP part.
 			// We should indicate this to the caller, as