[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

internal/email/signup.go

@@ -0,0 +1,42 @@
+// 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/>.
+
+package email
+
+var (
+	newSignupTemplate = "email_new_signup.tmpl"
+	newSignupSubject  = "GoToSocial New Sign-Up"
+)
+
+type NewSignupData struct {
+	// URL of the instance to present to the receiver.
+	InstanceURL string
+	// Name of the instance to present to the receiver.
+	InstanceName string
+	// Email address sign-up was created with.
+	SignupEmail string
+	// Username submitted on the sign-up form.
+	SignupUsername string
+	// Reason given on the sign-up form.
+	SignupReason string
+	// URL to open the sign-up in the settings panel.
+	SignupURL string
+}
+
+func (s *sender) SendNewSignupEmail(toAddresses []string, data NewSignupData) error {
+	return s.sendTemplate(newSignupTemplate, newSignupSubject, data, toAddresses...)
+}