Annotation count mismatch between sidebar and header in group activity page

### Steps to reproduce
1. Go to https://qa.hypothes.is/groups/__world__/public
2. Compare the count of annotations at the top of the screen and in the sidebar

### Expected behaviour
The two counts should match if there are no search terms other than the group to filter by.

### Actual behaviour
The counts for the public channel differ by 58246 (465575 vs 523821)

### Additional details
If it helps the implementation at all, I think it would be fine for the counts to be approximate once over a certain value, like mailbox counts are in Gmail. The more important thing is that they should be the same to avoid users wondering where the "missing" annotations are.