[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/index_register.tmpl

@@ -0,0 +1,41 @@
+{{- /*
+// 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/>.
+*/ -}}
+
+{{- define "registrationLimits" -}}
+New account registration is currently&nbsp;
+{{- if .instance.Registrations -}}
+    <b>open</b>.
+{{- else -}}
+    <b>closed</b>.
+{{- end -}}
+{{- end -}}
+
+{{- with . }}
+<section class="about-section" role="region" aria-labelledby="signup">
+    <h3 id="signup">Register an Account on {{ .instance.Title -}}</h3>
+    <div class="about-section-contents">
+        <p>{{- template "registrationLimits" . -}}</p>
+        {{- if .instance.Registrations }}
+        <p>To register a new account, please first read the <a href="/about#rules">rules</a> and <a href="/about#terms">terms</a>.</p>
+        <p>Then, use the <a href="/signup">sign-up page</a> to register an account.</p>
+        <p>Manual admin approval is <b>required</b> for new accounts.</p>
+        {{- end }}
+    </div>
+</section>
+{{- end }}