Feature request: Akkoma bubble timeline

[gerwitz]

The Akkoma backend offers a timeline called "bubble" (this is a configured aggregation of local timelines from a few designated instances).

I'd like to access this timeline from Pinafore.

[easrng]

I implemented this in my fork but my fork is kinda a mess so it might take some work to upstream it. It wasn't a very complicated process though, the most complicated part was making it fetch nodeinfo to check if it's supported/enabled (i just added it to the instanceinfo so i didn't need to add another store entry).