[feature] Allow admins to send test emails (#1620)

* [feature] Allow admins to send test emails

* implement unwrap on new error type

* add + use gtserror types

* GoToSocial Email Test -> GoToSocial Test Email

* add + use getInstance db call

* removed unused "unknown" error type

internal/email/test.go

@@ -0,0 +1,58 @@
+// 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
+
+import (
+	"bytes"
+	"net/smtp"
+
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+)
+
+const (
+	testTemplate = "email_test_text.tmpl"
+	testSubject  = "GoToSocial Test Email"
+)
+
+type TestData struct {
+	// Username of admin user who sent the test.
+	SendingUsername string
+	// URL of the instance to present to the receiver.
+	InstanceURL string
+	// Name of the instance to present to the receiver.
+	InstanceName string
+}
+
+func (s *sender) SendTestEmail(toAddress string, data TestData) error {
+	buf := &bytes.Buffer{}
+	if err := s.template.ExecuteTemplate(buf, testTemplate, data); err != nil {
+		return err
+	}
+	testBody := buf.String()
+
+	msg, err := assembleMessage(testSubject, testBody, toAddress, s.from)
+	if err != nil {
+		return err
+	}
+
+	if err := smtp.SendMail(s.hostAddress, s.auth, s.from, []string{toAddress}, msg); err != nil {
+		return gtserror.SetType(err, gtserror.TypeSMTP)
+	}
+
+	return nil
+}