Support for loading mp4 variant of i.reddit gifs

[scria1000]

No more waiting for 50-100mb gifs to load. Particularly useful for subreddits like r/educationalgifs

Works with galleries. Works with captions. Works with autoplay if it's not a gallery (still works if you open the gallery). You can download the original gif should you please, through a three dot menu (also present on Bottom Toolbar). I opted for hiding playback controls for gifs inside galleries to make room for captions (it's also what the official app does), maybe we can do it differently.

Maybe closes #515

[Turhvjbufv]

Any updates?