Remove finmoji and add a way to download emojis in packs

The metadata is currently stored at https://git.pleroma.social/vaartis/emoji-index.

Included is a mix task to list the packs on the server and install a pack from the server. I've made it somewhat fancy with ANSI color codes (only used if supported). Please suggest functionality to add to the task, if anything important is missing.

I've also created a pack for finmoji and deleted them from the main repository.

How it works:

• The task fetches the manifest
• The pack name is looked up in the manifest
• The pack is downloaded
• Only files specified in the manifest are unzipped into the static_dir/emoji/pack_name
• An emoji.txt file is created in the pack directory, it maps the shortcodes to real emoji paths.
• This file is used when initializing to load emojis into ETS.

TODO:

• Fetching multiple packs at once
• Providing a custom manifest
  • Allow the custom manifest to be a local file
• Allow local packs that don't have emoji.txt (assume all png files are emojis, for local custom emojis)
• Separate the index and the file list
• Add a way to verify assets (a hash of the source archive)
• Add a task to generate packs

Closes #821 (closed)

Closes #817 (closed) (there's a blobmoji pack now)