* Support filtering annotations by display name

Allow `user:{term}` filters in the search bar to match display names as
well as usernames.

I have allowed for annotations having no `user_info` field as with other
code in the client that handles annotation objects. However "h" will always
populate that field currently.

Fixes https://github.com/hypothesis/product-backlog/issues/943

* Add tests to clarify how quotes are handled in filter query parsing

This came out of a Slack discussion [1] about how the client handles
quoted and unquoted terms in filter queries.

[1] https://hypothes-is.slack.com/archives/C4K6M7P5E/p1549466094062700

* Test that all matching annotations are returned for "user" queries

Respond to CR feedback to extend the current set of tests for "user"
queries.