Add preference to enable/disable video looping

[cliambrown]

As requested in issue #1075
(However, this PR does not distinguish between videos & gifs)

Note: Very sorry but I was not able to test this because I do not have a Reddit API key.

• Adds pref_behaviour_video_loop preference (default = true to match current behaviour) with a corresponding checkbox on the "Images/Video" settings page
• If pref_behaviour_video_loop is true, the Player.REPEAT_MODE_ONE constant is used in ExoPlayerWrapperView.java as added in PR Make videos loop more smoothly #925
• If false, the Player.REPEAT_MODE_OFF ("Normal playback without repetition", docs) is used instead

[cliambrown]

Not sure the FeatureFlagHandler.java changes are really necessary, but I included them just in case.