fixed installer - ispconfig/ispconfig3#3750

Fixes #3750 

See merge request !277

- hooked ssl and ssl_letsencrypt checkboxes

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

missing menu for ftp stats

Hi, 

I forgot to send /interface/web/sites/lib/module.conf.php.
Now the ftp stat can be accessible by interface


See merge request !280

fix some bugs in commit f1926a01 - #3716



See merge request !278