First part of fix for issue #3896 - Unknown column 'default_mailserver' in 'field list' when saving client limits main template

please review the change!

Fixed issue #3955 DNS module: Limit for maximum zones can be circumvented with "import zone file" function.
Removed TODO string in error template.

Prevent 60 seconds delay in mailbox creation when mapping of mailbox zui and gid to web uid and gid is not used.

This reverts commit bb664a22.

show error-message when a client / reseller change the ip for a domain that was created by the admin

Proposed fix for #3929 (letsencrypt can not be activated when a new website gets created). Needs testing.