Modules JSON web service: generate static json file to download

[jmartsch]

It would be nice if a static file allmodules.json would be regenerated every time a module is updated/added to the modules directory.

This would enable us to query the file and check the ETag if it was changed.
The last changed information can be used to decide for my ModulesManger2 module if the local cached version of the module information should be used, or a new version should be pulled from the web.