Remove unused variables
Introduce pattern capture filter for user field.

This way we can search both for the username and the full user format.

Fix #1456
Get rid of the exact_match configuration parameter

As now we only use it with a false parameter every time,
it is no longer serving any purpose
Rename username ES filter to user
Merge pull request #1481 from hypothesis/1456-fix-user-search-2

Search for usernames