Fix issue that PHP version of a site gets altered when an admin edits the site when domain limit mode is active.

- removed master_directive_snippet code that was (in 2016) accidently backported from master but never used

global option to enable pagespeed with nginx - allows to enable pagespeed even if no webserver-config is selected

- fixed missing language file entries

- hooked ssl and ssl_letsencrypt checkboxes

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

- removed most static content from scrigo

- 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.