activity: don't mark read if tab not focused and automatically mark read at chat bottom

[arthyn]

This didn't have an issue, but it was old behavior we had that I think is better than what's currently in there. Basically there's three situations:

• tab is out of focus, so don't do any read marking and let them accumulate
• tab is in focus, and we're at the bottom of the channel, automatically mark read w/o showing markers
• tab is in focus, but we're not at the bottom of the channel, don't automatically mark read, let the normal "seeing" trigger mark read

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

[jamesacklin]

Cool, this makes sense. No need to flash the blue separator if you're actively watching the channel in focus.

[patosullivan]

Looks good, makes sense.