Refactored mail backends.

We now use Net::SSLGlue::POP3 and Net::SSLGlue::SMTP which allow
us to use the same base modules for the SSLified backends as for
the 'plain' backends.

Therefore we drop the following modules:

 - Net::SMTP::SSL
 - Net::SMTP::TLS::ButMaintained
 - Mail::POP3Client
 - Net::POP3::SSLWrapper

Because we now use similar mail libraries for the connections, we can
use much less code; and we use 'use base' to derive the content of
the SMTP, IMAP, and POP3 backends in the SSLified versions.

Another benefit of using the Net::SSLGlue modules is that we can force
SSL certificate verification to 'off' - or we can pass verification
options to IO::Socket::SSL if we want to do this in the future.
Passing full options to IO::Socket::SSL was previously not possible
with all mail modules.
Updated the save widget for the process edit screen.
Fixed error on Template using plain text.
Added support for templates in Ticket Phone Inbound and Outbound.
Added widgets to the PostMaster Filter module.
Merge branch 'master' of https://github.com/OTRS/otrs into master-nilei-AdminPostMasterFilterWidgets
Removed signature form Phone Inbound and Outbound templates.
Added missing entry.
Fixed bug#9666 Installing an invalid package cause an server error on package manager.
Extended fix for bug#9666 Installing an invalid package cause an server error on package manager.
Merge branch 'master-nilei-AdminPostMasterFilterWidgets' of https://github.com/nilei/otrs
Small fixes.
Merge branch 'master-nlei-ProcessManagement-ProcessEditSaveWidget' of https://github.com/nilei/otrs
Merge branch 'feature-mailrefactoring-mb-master' of https://github.com/mbeijen/otrs
Added changes entry.