Add support for turning custom stickers into URLs (e.g. larger custom emoji)

[digitalcircuit]

Discord recently (citation needed) added support for custom stickers, which appear to be larger versions of custom emoji. It's in the API documentation, too.

bitlbee-discord currently ignores these messages entirely, resulting in losing context when some folks choose to respond with stickers rather than text.

(I've been meaning to make a thorough bug report for a while, but until time/energy permits I figured I should file something then improve this later.)