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

doc/email-configuration.markdown

@@ -20,9 +20,7 @@ There are several email transports available:
 - SMTP
 - Sendmail
 - PHP native mail function
-- Mailgun
-- Postmark
-- Sendgrid
+- Other methods can be provided by external plugins: Postmark, Sendgrid and Mailgun
 
 Server settings
 ---------------
@@ -77,63 +75,6 @@ This is the default configuration:
 define('MAIL_TRANSPORT', 'mail');
 ```
 
-### Mailgun HTTP API
-
-You can use the HTTP API of Mailgun to send emails.
-
-Configuration:
-
-```php
-// We choose "mailgun" as mail transport
-define('MAIL_TRANSPORT', 'mailgun');
-
-// Mailgun API key
-define('MAILGUN_API_TOKEN', 'YOUR_API_KEY');
-
-// Mailgun domain name
-define('MAILGUN_DOMAIN', 'YOUR_DOMAIN_CONFIGURED_IN_MAILGUN');
-
-// Be sure to use the sender email address configured in Mailgun
-define('MAIL_FROM', 'sender-address-configured-in-mailgun@example.org');
-```
-
-### Postmark HTTP API
-
-Postmark is a third-party email service.
-If you already use the Postmark integration to receive emails in Kanboard you can use the same provider to send email too.
-
-This system use their HTTP API instead of the SMTP protocol.
-
-Here are the required settings for this configuration:
-
-```php
-// We choose "postmark" as mail transport
-define('MAIL_TRANSPORT', 'postmark');
-
-// Copy and paste your Postmark API token
-define('POSTMARK_API_TOKEN', 'COPY HERE YOUR POSTMARK API TOKEN');
-
-// Be sure to use the Postmark configured sender email address
-define('MAIL_FROM', 'sender-address-configured-in-postmark@example.org');
-```
-
-### Sendgrid HTTP API
-
-You can use the HTTP API of Sendgrid to send emails.
-
-Configuration:
-
-```php
-// We choose "sendgrid" as mail transport
-define('MAIL_TRANSPORT', 'sendgrid');
-
-// Sendgrid username
-define('SENDGRID_API_USER', 'YOUR_SENDGRID_USERNAME');
-
-// Sendgrid password
-define('SENDGRID_API_KEY', 'YOUR_SENDGRID_PASSWORD');
-```
-
 ### The sender email address
 
 By default, emails will use the sender address `notifications@kanboard.local`.