Add team member invite by email (#20307)

Allows to add (not registered) team members by email.

related #5353

Invite by mail:

![grafik](https://user-images.githubusercontent.com/1666336/178154779-adcc547f-c0b7-4a2a-a131-4e41a3d9d3ad.png)

Pending invitations:

![grafik](https://user-images.githubusercontent.com/1666336/178154882-9d739bb8-2b04-46c1-a025-c1f4be26af98.png)

Email:

![grafik](https://user-images.githubusercontent.com/1666336/178164716-f2f90893-7ba6-4a5e-a3db-42538a660258.png)

Join form:

![grafik](https://user-images.githubusercontent.com/1666336/178154840-aaab983a-d922-4414-b01a-9b1a19c5cef7.png)

Co-authored-by: Jack Hay <jjphay@gmail.com>

services/mailer/mail_release.go

@@ -23,7 +23,7 @@ const (
 	tplNewReleaseMail base.TplName = "release"
 )
 
-// MailNewRelease send new release notify to all all repo watchers.
+// MailNewRelease send new release notify to all repo watchers.
 func MailNewRelease(ctx context.Context, rel *repo_model.Release) {
 	if setting.MailService == nil {
 		// No mail service configured

services/mailer/mail_team_invite.go

@@ -0,0 +1,62 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package mailer
+
+import (
+	"bytes"
+	"context"
+
+	org_model "code.gitea.io/gitea/models/organization"
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/base"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/templates"
+	"code.gitea.io/gitea/modules/translation"
+)
+
+const (
+	tplTeamInviteMail base.TplName = "team_invite"
+)
+
+// MailTeamInvite sends team invites
+func MailTeamInvite(ctx context.Context, inviter *user_model.User, team *org_model.Team, invite *org_model.TeamInvite) error {
+	if setting.MailService == nil {
+		return nil
+	}
+
+	org, err := user_model.GetUserByIDCtx(ctx, team.OrgID)
+	if err != nil {
+		return err
+	}
+
+	locale := translation.NewLocale(inviter.Language)
+
+	subject := locale.Tr("mail.team_invite.subject", inviter.DisplayName(), org.DisplayName())
+	mailMeta := map[string]interface{}{
+		"Inviter":      inviter,
+		"Organization": org,
+		"Team":         team,
+		"Invite":       invite,
+		"Subject":      subject,
+		// helper
+		"locale":    locale,
+		"Str2html":  templates.Str2html,
+		"DotEscape": templates.DotEscape,
+	}
+
+	var mailBody bytes.Buffer
+	if err := bodyTemplates.ExecuteTemplate(&mailBody, string(tplTeamInviteMail), mailMeta); err != nil {
+		log.Error("ExecuteTemplate [%s]: %v", string(tplTeamInviteMail)+"/body", err)
+		return err
+	}
+
+	msg := NewMessage([]string{invite.Email}, subject, mailBody.String())
+	msg.Info = subject
+
+	SendAsync(msg)
+
+	return nil
+}