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.

- hooked ssl and ssl_letsencrypt checkboxes

- added limit for letsencrypt and directive snippets selector to client limits
- hide ssl tab if letsencrypt is selected

- Directive snippets can now be made available to clients; clients can select an available directive snippet for a website - the appropriate snippet will then be written to the vhost configuration file. This is useful for example on nginx where clients have no access to the Options tab of a website.