[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/media/media.go

@@ -18,24 +18,39 @@
 package media
 
 import (
+	"github.com/superseriousbusiness/gotosocial/internal/federation"
 	"github.com/superseriousbusiness/gotosocial/internal/media"
+	"github.com/superseriousbusiness/gotosocial/internal/processing/common"
 	"github.com/superseriousbusiness/gotosocial/internal/state"
 	"github.com/superseriousbusiness/gotosocial/internal/transport"
 	"github.com/superseriousbusiness/gotosocial/internal/typeutils"
 )
 
 type Processor struct {
+	// common processor logic
+	c *common.Processor
+
 	state               *state.State
 	converter           *typeutils.Converter
+	federator           *federation.Federator
 	mediaManager        *media.Manager
 	transportController transport.Controller
 }
 
 // New returns a new media processor.
-func New(state *state.State, converter *typeutils.Converter, mediaManager *media.Manager, transportController transport.Controller) Processor {
+func New(
+	common *common.Processor,
+	state *state.State,
+	converter *typeutils.Converter,
+	federator *federation.Federator,
+	mediaManager *media.Manager,
+	transportController transport.Controller,
+) Processor {
 	return Processor{
+		c:                   common,
 		state:               state,
 		converter:           converter,
+		federator:           federator,
 		mediaManager:        mediaManager,
 		transportController: transportController,
 	}