activity: fix notification paths

[arthyn]

Fixes TLON-2423 by adjusting the paths we send people to, reflecting the actual routes instead of all trying to be chats. Also fixed two issues encountered when testing.

• Creating a post and then commenting on it yourself was crashing because the source didn't exist
• Sometimes the feed would produce bundles where the latest time didn't match any event in the bundle, which makes no sense. So I changed it to always pull the "latest" value from the list of events it sends with the bundle (instead of the event on which we built this bundle which may not be present in the bundle itself)

PR Checklist

• Includes changes to desk files
• Describes how you tested the PR locally (test ship vs livenet)
• If a new feature, includes automated tests
• Comments added anywhere logic may be confusing without context

[linear]

TLON-2423 web: activity links to notebook comments fail

[latter-bolden]

js change makes sense

[Fang-]

A remaining nit, but looks good otherwise.