Video playback modal

Summary:
Bare-bones version of the video playback modal that will be completed in subsequent diffs.

Added video-playback-modal.react and modified app-navigator.react and route-names.

Test Plan: Tested basic functionality on iOS and android. Will test media pipeline scenarios comprehensively after the modal animation diffs.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, subnub, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D717