[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/account/account_test.go

@@ -111,7 +111,7 @@ func (suite *AccountStandardTestSuite) SetupTest() {
 	suite.emailSender = testrig.NewEmailSender("../../../web/template/", suite.sentEmails)
 
 	filter := visibility.NewFilter(&suite.state)
-	common := common.New(&suite.state, suite.tc, suite.federator, filter)
+	common := common.New(&suite.state, suite.mediaManager, suite.tc, suite.federator, filter)
 	suite.accountProcessor = account.New(&common, &suite.state, suite.tc, suite.mediaManager, suite.federator, filter, processing.GetParseMentionFunc(&suite.state, suite.federator))
 	testrig.StandardDBSetup(suite.db, nil)
 	testrig.StandardStorageSetup(suite.storage, "../../../testrig/media")