Add option to turn off swipe navigation between pages

[shambu2k]

A simple fix for issue #36. As mentioned, I have added an option to toggle the swipe to go back feature in the settings bottom sheet.
As of now, I have accessed the persistence provider in the app.dart file and passed the bool useGestureNavigation variable to the theme builders. If false, pageTransitionsTheme is set to null which turns off the Swipeable page transition.

[Mosc]

I had envisioned this as a setting that disables all gestures, including swipes on item tiles (stories, comments, etc.) to fix #32 at the same time. Do you think that makes sense?

[Mosc]

Thanks for the contribution!

[shambu2k]

I had envisioned this as a setting that disables all gestures, including swipes on item tiles (stories, comments, etc.) to fix #32 at the same time. Do you think that makes sense?

Hmm yeah, makes sense. I only thought of SwipeablePageTransition as I never had issues with upvote or reply gestures. Will try to implement for other gestures.

[shambu2k]

Now this fixes #32 too. Included option to toggle all gestures within the app in the settings bottom sheet.

[Mosc]

Nice job on the implementation overall, just some minor details I'd like to see changed to fit my currently preferred style.

[Mosc]

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[Mosc]

Nice!