Better plugin support

[thomasballinger]

Currently plugins work... so long as they're listed at https://github.com/EndlessSkyCommunity/endless-sky-plugins/blob/master/generated/plugins.json. Plugins are downloaded each time, there's no caching.

• maybe we should cache these (https://raw.githubusercontent.com/ is probably ok to depend on, but the CORS proxy I'm running not long-term sustainable)
  • or just change the CORS proxy behavior to be more specific to avoid abuse
• this index isn't very complete. Do people know how to add to this repository? Are the ES Launcher folks using this?
• specify a plugin set via url?

[thomasballinger]

What was just implemented for https://github.com/quyykk/editor and ported over in https://github.com/thomasballinger/endless-web/commit/2c94b6d4b94dd72cf23630b2a0222b24cf271122 seems better: now you can upload and download plugins too.

[thomasballinger]

Current plugin support: https://play-endless-web.com/?plugins

(FYI @deiniolte2376)