activity: refine combined count for channels

[arthyn]

After discussion with @latter-bolden, we decided to change how count in activity summaries gets tallied to better reflect what expect UX-wise, aka ignoring threads that don't involve us.

We refresh all summaries on load since that shouldn't take very long and likely should be done anytime the agent changes, and in this case we have to for those counts to get updated immediately.

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

[Fang-]

I can debate the expectation and its implications here, but the code seems to reflect the description at least, so looks good to me.

[arthyn]

Fixes TLON-2202