Improve routing in app

[tamslo]

• [x] Login should replace stack
• Drug selection
  • [x] Add "change later in settings or per drug" to description
  • [x] Pressing continue should overwrite stack
  • [x] Continue should only be possible in onboarding; pass down bool
• Onboarding
  • [x] It should be possible to navigate back (push next page)
  • [x] Next page should be possible to be null (to go back to settings); if null, text should be "Back to app"
  • [x] Last button should be elevated because swiping right not possible on last page
  • [x] Add property "dismissable" with "x" icon to navigate back
• [x] Add metadata checkpoints for onboardingDone and initialDrugSelectionDone; initial route of app should depend on this
• [x] Wrap root tab routes in WillPopScope (false) – test if going back still works for gene and drug detail (it should!)