Reflect and retain filter state in URL params.

This commits makes two important changes:
- Whenever we click on filter checkboxes, the URL is immediately updated
  to include parameters that represent the chosen options. For example,
  if you select Beginner complexity, the url is updated with
  ?complexity=beginner.
- Whenever the page is loaded, we deserialize any parameters that exist
  in the URL and change the filter state to match it. In other words,
  refreshing the page at any time retains the current state,
  and you can now link people to exactly the filter results that you
  want to share.

0 files changed, 0 insertions, 0 deletions