Provide more options for sorting groups activity

### Problem you are trying to address with this feature
As requested here: https://hypothesis.zendesk.com/agent/tickets/2145

Currently, annotations on groups pages are sorted by document and then by the order in which they were made. For education users, it's sometimes more useful to see group annotations by date rather than by document.

### Your solution
Jon's export utility allows for this, but it would be nice to make this a part of the product. Just as users can sort by users and tags, we could add an option for ascending/descending order of timestamps.

Start with the groups pages and the existing sorting controls for users and tags. Trace how annotations are currently ordered by document and creation order, then define the ascending and descending timestamp options. Done means users can sort group annotations by date in both directions, alongside the existing options.

Written by the indexing model from the issue text.