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/nb_NO/translations.php

@@ -353,7 +353,6 @@ return array(
     'Clone' => 'Kopier',
     'Project cloned successfully.' => 'Prosjektet er kopiert.',
     'Unable to clone this project.' => 'Prosjektet kunne ikke kopieres',
-    'Email notifications' => 'Epostvarslinger',
     'Enable email notifications' => 'Aktiver epostvarslinger',
     'Task position:' => 'Oppgaveposisjon:',
     'The task #%d have been opened.' => 'Oppgaven #%d er åpnet.',
@@ -730,17 +729,11 @@ return array(
     // 'This chart show the task complexity over the time (Work Remaining).' => '',
     // 'Screenshot taken %s' => '',
     'Add a screenshot' => 'Legg til et skjermbilde',
-    'Take a screenshot and press CTRL+V or ⌘+V to paste here.' => 'Ta et skjermbilde og trykk CTRL+V for å lime det inn her.',
+    // 'Take a screenshot and press CTRL+V or ⌘+V to paste here.' => '',
     'Screenshot uploaded successfully.' => 'Skjermbilde opplastet',
     // 'SEK - Swedish Krona' => '',
     'The project identifier is an optional alphanumeric code used to identify your project.' => 'Prosjektkoden er en alfanumerisk kode som kan brukes for å identifisere prosjektet',
     'Identifier' => 'Prosjektkode',
-    // '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' => '',