Support gif banners, fallback to static if not supported

[nyakase]

GIF banners should be allowed in bannerfest, but only if there is a static version of it to fallback to.
See https://discord-api-types.dev/api/discord-api-types-v10/enum/GuildFeature#AnimatedBanner.

• If the server has animated banner support, great, just set it.
• If the server has no animated banner support but does have banner support, set the static banner and post the animated variant in #dumpster.
• If the server has no banner at all, post the animated variant in #dumpster.