REST: Resolve performance issues with '/events' web view

REST: Resolve performance issues with '/events' web view

The dropdown select-based filters in the web view of the REST API have
stung us a few times. In this case, populating these filters for the
'/events' endpoint results in a huge query that hammers the database and
results in seriously laggy responses.

The root cause of this performance issues was erroneously identified as
an issue with the JSON renderer so that particular patch can now be
reverted. This will be done separately.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Cc: Daniel Axtens <dja@axtens.net>
Tested-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Daniel Axtens <dja@axtens.net>