This removes all AngularJS code
Angular version 1 is unmaintained and largely obsolete since years.
Additionally, I'd rather re-use the conventions and libraries from
Converse instead of having different ones in converse-desktop.
This means we're losing some functionality, such as the app settings
and the about modal.
This is unfortunate, but ideally (non-Electron) app settings should
be implemented in Converse itself and not in this repo.
