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.

[leonvogt]

I did not continue working on this PR since this comment in the iOS repo suggests that the behavior of historical_locations may change in the near future. (Like closing the modal, on /resume_historical_location)

Unless I hear otherwise, I'll wait for the changes in iOS before continuing work on this PR.

The only downside is that the current Android version will freeze when the server redirects to /recede_historical_location and a modal is present, as described in the issue.