Try to handle errors when connecting to amavis.

fixes #257
force html links to open in a new tab.
LDAP improvements:

* Support for domain adminstrator groups mapping (fixes #449)
* Support for better hash algorithm (see #450)
Fixing help text.

see #449
Fixing password verification for sha256.
Allowing default domain quota.

fixes #500
Merge branch 'master' into feature/score-and-sorting-in-quarantine
Fix activation issue.