Module manager

Adds a basic form to enable/disable optional Plinth modules under "System" > "Package Manager".
Currently the list of optional modules is stored in actions/module-manager as an associative array, with module names mapped to their install location under modules/installed/.

Bugs: When you disable the owncloud module, it will disappear from Plinth after about a minute. But when you re-enable the module, it won't reappear in Plinth, until Plinth is restarted.