Fixed/Implemented: replaced intval() by $app->functions->intval() in all interface functions due to big number problem in intval()

Implemented:  FS#2422 - Use domain module when creating subdomains 
Bugfix: datalog was used in the wrong way on editing aliasdomains
Bugfix: missing datalog update for parent domain on editing subdomains

1) Edit a domain: The selected domain is also selected in the dropdown-list
2) Edit a domain: The admin can now select any domain not only his/her

Fixed a bug: if the domain-module is used and there are NO domains in the list, then the user was able to free edit the domain-name. This bug is fixed now and a empty list is generated, so the user can enter only a empty name!

Added a new module called "domain". With this module you can manage all domains (web + mail). Per default this module is not active. To activate it, you have to change the System Interface Config -> domains -> use the domain module. You also have to activate the domain-module for the admin (and for every client if you want). Important: you can not use this module if you have reseller!

Additionally fixed a bug in web quota calculation.

Improved client and reseller limit checks

Fixed: FS#787 - A sub-client can see all the clients on the system in the client drop down list 

FS#665 - Reseller adding email domain 
FS#666 - Reseller adding mysql database 

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- Updated Debian installation instructions.

- Changed shell of website linux user to /bin/false

Added a server module for clients and a mail plugin.