Merge branch 'domain_specific_welcome_message' into 'stable-3.1'

Allow domain specific welcom messages:

* In addition to language specific custom welcome messages (when creating a new mail account) this patch will
allow to have a welcome message template for each domain.

* To add a domain specific welcome message, place a file at /usr/local/ispconfig/server/conf-custom/mail/
and name it according the domain, like welcome_email_example.com.txt

* i.e. /usr/local/ispconfig/server/conf-custom/mail/welcome_email_example.com.txt

will be used if a mailbox for example.com is created.

See merge request !263