activity: fix init unreads

[arthyn]

Fixes TLON-2447 by adding a new state transition that will run a fix. The fix looks for any DM or channel sources with only one event which should be either %dm-invite or %chan-init and sets the floor to the last-read time from the old agents. If this is actually an invite pending, then it will stay unread which is what we want. If it's not pending and they had in fact previously interacted and read that channel, then it will be marked read.

Unfortunately, to call these migration methods I had to factor them out to be pure so that we could call them on old state. Sync reads tests still pass, going to write some for this fix scenario.

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-2447 activity: bell has blue dot but no notifications

[Fang-]

This looks like what we reviewed out of band yesterday. Nothing serious, just some small clean-up things.