Enhance login view

[tobiasKaminsky]

This was split from #21.

• have a separate login user interface (independent from settings)
• use v2 login flow
• Resemble user interface from Nextcloud Files app
• Create screenshots for comparison before and after
  • Light appearance
  • Dark appearance
  • Landscape orientation
  • Portrait orientation
  • iPhone
  • iPad
  • Nextcloud Files and Nextcloud Talk for reference
  • Possibly new screenshots are necessary for the App Store)
• Based on what the Talk for iOS team does and the experience with automated UI tests in Nextcloud Files for iOS, include a launch argument parser for app tokens. This simplifies and accelerates future UI tests a lot by bypassing the login procedure in the web view when unnecessary.