Fix modal navigation

[leonvogt]

This PR attempts to solve the issue #84.

Problems solved:

• Modal closes after redirecting to /resume_historical_location
• Jump to the first fragment, when a modal was closed on the second fragment via /recede_historical_location

One left inconsistency between Android and iOS is that:

• After the modal is popped, through /recede_historical_location, Android requests the location of the previous fragment. iOS doesn't do a request.
  Seems to me, that happens because the navigator always request the location of the newly attached fragment in: HotwireWebFragmentDelegate.kt#328
  Not sure how to proceed here.

Another thing is, that the app previously freezed when the modal was popped through /recede_historical_location on the first fragment (as mentioned in the issue). I've fixed that by setting shouldNavigate to false, if we are popping a fragment.
But i guess it could be worth to investigate more, why the app freezed in the first place.