Make mail transports pluggable and move integrations to plugins

- Postmark: https://github.com/kanboard/plugin-postmark
- Mailgun: https://github.com/kanboard/plugin-mailgun
- Sendgrid: https://github.com/kanboard/plugin-sendgrid

app/Locale/th_TH/translations.php

@@ -353,7 +353,6 @@ return array(
     'Clone' => 'เลียนแบบ',
     'Project cloned successfully.' => 'เลียนแบบโปรเจคเรียบร้อยแล้ว',
     'Unable to clone this project.' => 'ไม่สามารถเลียบแบบโปรเจคได้',
-    'Email notifications' => 'อีเมลแจ้งเตือน',
     'Enable email notifications' => 'เปิดอีเมลแจ้งเตือน',
     'Task position:' => 'ตำแหน่งงาน',
     'The task #%d have been opened.' => 'งานที่ #%d ถุกเปิด',
@@ -735,12 +734,6 @@ return array(
     // 'SEK - Swedish Krona' => '',
     // 'The project identifier is an optional alphanumeric code used to identify your project.' => '',
     // 'Identifier' => '',
-    // 'Postmark (incoming emails)' => '',
-    // 'Help on Postmark integration' => '',
-    // 'Mailgun (incoming emails)' => '',
-    // 'Help on Mailgun integration' => '',
-    // 'Sendgrid (incoming emails)' => '',
-    // 'Help on Sendgrid integration' => '',
     // 'Disable two factor authentication' => '',
     // 'Do you really want to disable the two factor authentication for this user: "%s"?' => '',
     'Edit link' => 'แก้ไขลิงค์',