Add settings page to configure invidious instance

Fixes #25.

Screenshot: https://invent.kde.org/plasma-mobile/plasmatube/uploads/227a43b6af34c88d90a5029813342382/grafik.png

Currently, this only replaces the hard coded URL from `constants.h`
(restart required to take effect). It should be unified with the
account handling. However, the implementation works so that can be done
later on.