Phase out manual avatar entry

[ErikBoesen]

After seeing #196, I realized it might be a good idea to totally phase out the process of manually PR-ing avatars into this repository since FIRST maintains avatars themselves now which are often just as good quality if not better. It might help if we could get a count of how many teams who have submitted avatars to this repo also have official FIRST avatars—my guess is that most teams who've submitted here have submitted more professional official avatars. If only they'd stop submitting black or white backgrounds.

[EthShaw]

I just did some tests:

152/188 (~81%) teams with manually submitted avatars also have a FIRST avatar, which means
36/188 (~19%) do not.

I threw together a comparison of the avatars of teams that have both manually submitted logos and official FIRST avatars here. Manually submitted avatars are on the left, official FIRST avatars are on the right. A lot of teams' official FIRST avatars do seem to be better / higher quality than their uploaded avatars, though some do have added black/white backgrounds in their official FIRST avatars.

[ErikBoesen]

@FishDog5000 I'm entering college and will likely have very little time to look into this, so do whatever you think is best! :)