Add classifiers to search

[di]

This PR addresses the "faceting" part of #702 by adding the PyPI classifiers to the search page.

A few notes about things I was unsure of:

• Multiple classifiers act as a union (not an intersection). Not sure if this was the original intention.
• Selecting a checkbox for a classifier immediately submits the form. I wasn't sure how the UI should work here without introducing a lot of AJAX complexity, and it didn't feel intuitive to have the user scroll back up to the "Search" button to submit the form.
• A new search from the search bar currently clears the classifiers (and the ordering) thus starting a new search, but this can be easily changed.

Things I am leaving for a future PR (or for @nlhkabu):

• Only way to remove classifiers is unchecking their checkbox. The "X" box on the filter is not wired up.
• Classifiers are just nested/indented with multiple  s
• Make trove classifiers accordion only open one tab at once #755 is still valid

[nlhkabu]

This is so exciting! Thankyou @di! Give me a couple of days to pull it down and play with the UI. Ping @dstufft for a code review.

[dstufft]

Code wise this looks good to me, so as long as @nlhkabu is happy with the design side it can merge IMO.

[nlhkabu]

ok for me - @dstufft can you please rebase and merge?

[dstufft]

@di will need tod o the rebasing unless I make a new PR, which I can do if he'd rather that.

[di]

I'll rebase tomorrow AM, thanks!

[di]

@dstufft Good to go.

[dstufft]

Awesome, thanks!