Tracking: log search mode
Created by: attfarhan
Fixes https://github.com/sourcegraph/sourcegraph/issues/7831.
This PR adds logging for when users toggle search modes (and includes which mode they are toggling to), and adds a mode:
field specifying which search mode was used to the SearchResultsQueried
event. I'm not sure if this is the correct way to send this data (cc @ebrodymoore), let me know if it should be changed.