OAuth Support (#53)

Rips out the old auth method, adds OAuth support. Fixes #51.

Tested Platforms:
- [x] Linux
    - [x] Flatpak
    - [x] Native (Nix)
- [x] Android
- [ ] Windows
- [ ] MacOS

Reviewed-on: Nexus/nexus#53

ios/.gitignore

@@ -1,34 +0,0 @@
-**/dgph
-*.mode1v3
-*.mode2v3
-*.moved-aside
-*.pbxuser
-*.perspectivev3
-**/*sync/
-.sconsign.dblite
-.tags*
-**/.vagrant/
-**/DerivedData/
-Icon?
-**/Pods/
-**/.symlinks/
-profile
-xcuserdata
-**/.generated/
-Flutter/App.framework
-Flutter/Flutter.framework
-Flutter/Flutter.podspec
-Flutter/Generated.xcconfig
-Flutter/ephemeral/
-Flutter/app.flx
-Flutter/app.zip
-Flutter/flutter_assets/
-Flutter/flutter_export_environment.sh
-ServiceDefinitions.json
-Runner/GeneratedPluginRegistrant.*
-
-# Exceptions to above rules.
-!default.mode1v3
-!default.mode2v3
-!default.pbxuser
-!default.perspectivev3