[feature] Start implementing refetch of lost media files via /api/v1/admin/media_refetch (#1221)

* [chore] Move ShortcodeDomain to its own little util func

* [feature] Add RefetchEmojis function to media manager

* [feature] Expose admin media refresh via admin API

* update following review feedback

- change/fix log levels
- make sure not to try to refetch local emojis
- small style refactoring + comments

* log on emoji refetch start

Signed-off-by: kim <grufwub@gmail.com>
Co-authored-by: kim <grufwub@gmail.com>

internal/processing/admin/mediarefetch.go

@@ -0,0 +1,48 @@
+/*
+   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 admin
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/gtsmodel"
+	"github.com/superseriousbusiness/gotosocial/internal/log"
+)
+
+func (p *processor) MediaRefetch(ctx context.Context, requestingAccount *gtsmodel.Account, domain string) gtserror.WithCode {
+	transport, err := p.transportController.NewTransportForUsername(ctx, requestingAccount.Username)
+	if err != nil {
+		err = fmt.Errorf("error getting transport for user %s during media refetch request: %w", requestingAccount.Username, err)
+		return gtserror.NewErrorInternalError(err)
+	}
+
+	go func() {
+		log.Info("starting emoji refetch")
+		refetched, err := p.mediaManager.RefetchEmojis(context.Background(), domain, transport.DereferenceMedia)
+		if err != nil {
+			log.Errorf("error refetching emojis: %s", err)
+		} else {
+			log.Infof("refetched %d emojis from remote", refetched)
+		}
+	}()
+
+	return nil
+}