Support guest access token routing in Flutter

[mpge]

Summary

• add guest access token parsing to the Ticket model
• route guest ticket creation through the access token while still displaying the human ticket reference
• update the intl constraint for current Flutter stable and fix CardThemeData compatibility
• add unit coverage for guest route reference behavior

Verification

• flutter pub get
• flutter test
• flutter analyze --no-fatal-infos --no-fatal-warnings