[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/cache/size.go

@@ -334,7 +334,6 @@ func sizeofEmoji() uintptr {
 		ImageStaticPath:        exampleURI,
 		ImageContentType:       "image/png",
 		ImageStaticContentType: "image/png",
-		ImageUpdatedAt:         exampleTime,
 		Disabled:               func() *bool { ok := false; return &ok }(),
 		URI:                    "http://localhost:8080/emoji/01F8MH9H8E4VG3KDYJR9EGPXCQ",
 		VisibleInPicker:        func() *bool { ok := true; return &ok }(),
@@ -473,12 +472,10 @@ func sizeofMedia() uintptr {
 		File: gtsmodel.File{
 			Path:        exampleURI,
 			ContentType: "image/jpeg",
-			UpdatedAt:   exampleTime,
 		},
 		Thumbnail: gtsmodel.Thumbnail{
 			Path:        exampleURI,
 			ContentType: "image/jpeg",
-			UpdatedAt:   exampleTime,
 			URL:         exampleURI,
 			RemoteURL:   exampleURI,
 		},