Player: Fix playback position of already watched videos

[Fijxu]

Closes #3976

Trying to watch an already watched video will make the video start 15 seconds before the end of the video. This is not very comfortable when listening to music or watching/listening playlists over and over. This can be easily tested on any instance with the "Save playback position" enabled on Preferences.

[pimlie]

Thanks for the fix. Wdyt about listening on the player ended event to reset the play position? Might be a bit cleaner solution then using a fixed threshold, but not sure how/where the playback position is recorded though and if there is an api endpoint available for that.

[SamantazFox]

Thanks for your PR!