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/zh_CN/translations.php

@@ -353,7 +353,6 @@ return array(
     'Clone' => '克隆',
     'Project cloned successfully.' => '成功复制项目。',
     'Unable to clone this project.' => '无法复制此项目',
-    'Email notifications' => '邮件通知',
     'Enable email notifications' => '启用邮件通知',
     'Task position:' => '任务位置：',
     'The task #%d have been opened.' => '任务#%d已经被开启.',
@@ -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' => '编辑链接',