[MM-32622] Remove app.WaitForChannelMembership() (#17048)

* Remove app.WaitForChannelMembership

* Fix tests

* Fix test

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/context.go

@@ -0,0 +1,15 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+import (
+	"context"
+
+	"github.com/mattermost/mattermost-server/v5/store/sqlstore"
+)
+
+// WithMaster adds the context value that master DB should be selected for this request.
+func WithMaster(ctx context.Context) context.Context {
+	return sqlstore.WithMaster(ctx)
+}