Count replies (not just annotations) next to usernames in group sidebar

These numbers next to usernames in the group sidebar:


only count annotations and highlights, not replies. [Feedback from a user](https://hypothesis.zendesk.com/agent/tickets/866):

> Am I right in assuming that the numbers of annotations for each member only includes the annotations that they originated, and not the replies they may have made to other members' annotations?
>
> This detail is important when my assignments are along the lines of: Create at least 2 annotations and reply to at least 1 peer's annotation. I'm wondering if it would eventually be possible to also search for and display these reply statistics.

seanh's thoughts: maybe it should be the number of annotations, highlights and replies combined, and then on hover a tooltip breaking it down into separate numbers for annotations, highlights and replies individually. A tooltip might not be the best way, but some way to drill into the sum number and see a breakdown.

No files, tests, or entry points are named. Start by locating the group sidebar implementation and the code that calculates the username counts; confirm the intended treatment of replies and whether a breakdown is required. Done should make reply activity visible alongside annotations and highlights, with the final presentation agreed.

Written by the indexing model from the issue text.