Makefile: Build ".pyo" files on "make install" (bug 1786)
Database: Check if every episode is assigned to a podcast
Model: Disallow subscribing to an already-existing URL
QML UI: Directly add a podcast via feed URL

Don't go through gpodder.net if the user enters a URL
directly into the search field. This also fixes various
bugs / annoyances related to adding/removing podcasts in
the QML UI (e.g. updating "All episodes" counts).
Merge remote-tracking branch 'upstream/master'