Add trivial conflict checking

This pull request add simple conflict checking to the web ui and to the HTSP server.

It works by retrieving all the scheduled recordings between 2 times and then attempting to allocate the channels that would be used by those recordings to devices. It doesn't currently take into account current state of the server (this could be added in the future).

I've tested this on a homogeneous setup where it will work best, however it should still work on setups that don't have multiplexes with the same services.