[feature] New user sign-up via web page (#2796)

* [feature] User sign-up form and admin notifs

* add chosen + filtered languages to migration

* remove stray comment

* chosen languages schmosen schmanguages

* proper error on local account missing

web/template/email_new_signup.tmpl

@@ -0,0 +1,32 @@
+{{- /*
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// 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/>.
+*/ -}}
+
+Hello moderator of {{ .InstanceName }} ({{ .InstanceURL }})!
+
+Someone has submitted a new account sign-up to your instance.
+
+They provided the following details:
+
+Email address: {{ .SignupEmail }}
+Username:      {{ .SignupUsername }}
+{{- if .SignupReason }}
+Reason:        {{ .SignupReason }}
+{{- end }}
+
+To view the sign-up, paste the following link into your browser: {{ .SignupURL }}