[feature] Fetch + display custom emoji in statuses from remote instances (#807)

* start implementing remote emoji fetcher

* update status where pk

* aaa

* tidy up a little

* check size limits for emojis

* thank you linter, i love you <3

* update swagger docs

* add emoji dereference test

* make emoji max sizes configurable

* normalize db.ErrAlreadyExists

internal/media/processingemoji.go

@@ -28,6 +28,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	"github.com/superseriousbusiness/gotosocial/internal/config"
 	"github.com/superseriousbusiness/gotosocial/internal/db"
 	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
 	"github.com/superseriousbusiness/gotosocial/internal/log"
@@ -170,6 +171,11 @@ func (p *ProcessingEmoji) store(ctx context.Context) error {
 		return fmt.Errorf("store: error executing data function: %s", err)
 	}
 
+	maxSize := config.GetMediaEmojiRemoteMaxSize()
+	if fileSize > maxSize {
+		return fmt.Errorf("store: emoji size (%db) is larger than allowed emojiRemoteMaxSize (%db)", fileSize, maxSize)
+	}
+
 	// defer closing the reader when we're done with it
 	defer func() {
 		if rc, ok := reader.(io.ReadCloser); ok {