Merge branch 'feature/imap4flags' into 'master'

Support for "Mark as read" in user's mail filters

A few people on the internet have already requested support for the `imap4flags`. This pull request provides two things:

 - The templates for Sieve have been updated to include and load the `imap4flags` module. This is available also for Doctrine 1.1 and provides the ability to use the command `setflat "\\Seen";` which marks mails as read.
 - The web interface has been updated for a new action beside delete and more, named "Mark as read". This allows users to define mail filters which mark mails automatically as read. This currently only works for Sieve. Servers using Maildrop will silently ignore that specific user filter.

I would be happy to see that feature in ISPConfig 3.1!

See merge request !402