Allow website for certain usergroups only

[DiogoParrinha]

Restrict the website field to certain groups only, i.e. by default it is enabled but admins should be able to turn it off for certain groups.

[Sama34]

I can do this on #900, I could add a groupselect setting for each field.

[DiogoParrinha]

@Sama34 not necessary, I'll do this :)

[DiogoParrinha]

Merged my PR, everything seemed to work fine for me.

[Destroy666x]

If a user adds/changes website before his group is disallowed to do that, it's still displayed in posts/profile. usergroup['canchangewebsite'] checks should be added there like for signature (in posts at least).

BTW, signature usergroup permissions are checked in posts only, a check should be added to member profile too.

[DiogoParrinha]

Yeah you're right, I need to add that check for profile and postbit.

As for the signature permissions, they're not related to this issue.