Install hook for Apps

[peakwinter]

Currently there is no way for an app to perform specific actions right after it has been installed, save for 1) a hacky use of Signals, or 2) the creation of a PKGBUILD. And with both of those options, there is no way to provide custom information to the app as it is being installed and/or loaded for the first time, like there is upon website creation.

Ideally there would be an on_install.py in the base of an application that is executed after the app is successfully installed and loaded for the first time. This script could be provided with extra_vars, much like websites are now. This would eliminate the need for hacky things like what is being done presently when the Radicale app is initialized.