Feature: mp4 support

[hobgoblina]

Before You Begin

• I have checked for similar feature requests in the project's issue tracker
  to avoid duplicates.
• I have searched existing issues to see if this feature has been discussed
  before.

Feature Details

mp4 support would be dope. this was requested in #34 but they didn't try to justify it.

Is your feature request related to a problem?

when i try to use mp4 files with ewe, it replaces the video with a static image of the first frame.

mp4 files are much more likely to be high quality compared with gifs.

these days, i'm actually coming across mp4s more often than gifs because of the higher quality. e.g., a significant amount of the files supplied by giphy are mp4s these days.

additionally, it's actually easier to use mp4s with certain services. e.g., on mastodon i often have issues posting gifs and have to convert them to mp4s. this also means that the "gifs" that other people post, which I may want to save and add to my ewe collection, are most likely mp4s.

Describe the solution you'd like

support mp4 video files. maybe you could impose a file size limit, if that would help make the implementation easier? all the mp4s i'm trying to add are less than half a MB, even when they're ~15 seconds long and fairly high res (compared to gifs at least... no visible artifacts on phones)

Describe alternatives you've considered

converting to gifs, which is a real pain and results in significantly lower quality unless i go through a significant amount of effort to do it via gimp/etc, which nobody has time for when you just wanna save a reaction gif

Additional Context