Convert twemoji national flags into local assets

[BGMP]

These are currently served remotely, which is rather slow and might cause problems down the line if the links ever stop working.

We should get each one of the images into a flags/ directory or something... that aside, some national flags are also missing; we must run some tests to figure out which ones specifically though.

Reference: https://github.com/Re-Volt-America/RVA/blob/master/app/assets/stylesheets/twemoji/flags.scss