When searching for multiple groups, allow for use of OR instead of AND

## User story
As a Hypothesis-on-the-web user, I want to run a query at https://hypothes.is/search that shows me annotations from multiple groups.

## Additional information
As [discussed in Slack](https://hypothes-is.slack.com/archives/C2BLQDKHA/p1565962785142300), it would be nice to enable OR searches for `group` queries.

Presently, queries are joined by AND so that each additional term narrows my search:

(Resolves to https://hypothes.is/search?q=group%3AMndm6GvX+group%3A62V5MKkV)

This doesn't really make sense for group queries, since an annotation can't possibly exist in group1 AND group2.

Whether we'd want to _default_ to use of OR for group queries or simply _allow_ for it is a technical design question that could be addressed when it's time to propose an implementation.

EDIT: Adding [a ticket from Zendesk](https://hypothesis.zendesk.com/agent/tickets/5629) .- where a user was trying to do something like:

user:me OR group:a OR group:b etc...

Reason:
User wants to see his personal notes and the notes of anyone in any groups he's in.

Also, when this change is made we should update any dev + user facing docs to reflect this.