This implements ispconfig/ispconfig3#3644

This Checkbox can be enabled or disabled by template.

Further I fixed an inconsistent design (HTML) which was introduced by @tbrehm yesterday I tink. (DKIM-Checkbox)

if the filter rule uses "begins with", the regex should be "^$searchterm" instead of " $searchterm"

- ignore limit_client from templates if they don't match client type (client/reseller) (Fixes #3734)