Push requests for install/uninstall

This is the first basic implementation of the idea of "push requests" from the server. This implements two requests that the server can ask of all clients: install and uninstall. A default repository in the client can be marked to ignore or always accept push requests in this merge request. There is also the sketch of a third option called prompt which gives the user the standard "Just Once/Always" choice; that is not yet implemented.

This allows central management of app installs/uninstalls for a pool of devices, as well as other ideas.

A use case for this feature is documented here: https://f-droid.org/wiki/page/Whitelabel_Builds