AppView: support "undelete" repo, ie undo tombstone

[snarfed]

Hi all! @bnewbold and I talked about this on and off a while back, and @ericvolp12 and I just now in Discord, but I couldn't find an open issue to track it, either here or in the indigo repo, so I figured I'd start one. Feel free to move or redirect if you want.

I currently emit #tombstone events over subscribeRepos when accounts on my PDS are disabled/deleted. I plan to switch to #account events eventually, https://github.com/snarfed/bridgy-fed/issues/1119 , but I've waited because I want to be able to undo a tombstone and bring a repo back to life. It sounds like the AppView doesn't support this yet, but I'm not sure if you expect to eventually.

@bnewbold thought that emitting #identity + #account with active: true might do it, but I tried that, and it doesn't work quite yet: https://github.com/snarfed/bridgy-fed/issues/1130#issuecomment-2248531340. More background: https://github.com/snarfed/bridgy-fed/issues/1119#issuecomment-2233362196

Thanks in advance!