[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/federation/dereferencing/emoji.go

@@ -0,0 +1,51 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package dereferencing
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/url"
+
+	"github.com/superseriousbusiness/gotosocial/internal/media"
+)
+
+func (d *deref) GetRemoteEmoji(ctx context.Context, requestingUsername string, remoteURL string, shortcode string, id string, emojiURI string, ai *media.AdditionalEmojiInfo) (*media.ProcessingEmoji, error) {
+	t, err := d.transportController.NewTransportForUsername(ctx, requestingUsername)
+	if err != nil {
+		return nil, fmt.Errorf("GetRemoteEmoji: error creating transport: %s", err)
+	}
+
+	derefURI, err := url.Parse(remoteURL)
+	if err != nil {
+		return nil, fmt.Errorf("GetRemoteEmoji: error parsing url: %s", err)
+	}
+
+	dataFunc := func(innerCtx context.Context) (io.Reader, int, error) {
+		return t.DereferenceMedia(innerCtx, derefURI)
+	}
+
+	processingMedia, err := d.mediaManager.ProcessEmoji(ctx, dataFunc, nil, shortcode, id, emojiURI, ai)
+	if err != nil {
+		return nil, fmt.Errorf("GetRemoteEmoji: error processing emoji: %s", err)
+	}
+
+	return processingMedia, nil
+}