[bug 1006574] Initial implementation of top contributors in the community hub.

This uses the data we started indexing a few weeks ago to get the top contributors per area of the site. It makes a facet query to ES for that. We show the top 10 contributors per area on the community hub landing page (/community) and then have a longer list if you click the "See all" link.

There is a lot more to add, but I think it's at a good check point to review and land and keep going. Some thing that we probably need to file bugs for:
* Add pagination to the top contributors list. Right now I just cut them off at 50.
* Show more data about the user in the result card. Some things they want are "Last activity date" and "# of solutions" in the case of Support Forum.
* Add some filters to the top contributor lists. L10n should have a locale dropdown filter, for example. Questions probably should have one too. We could possibly add product filters too.
* Go through and figure out what to do to all the HTML I commented out.

To verify:
* run tests
* look at /community
* look at /community/army-of-awesome, /community/kb, /community/l10n, /community/questions
(For those last two, you need to have all the contribution data indexed in ES. I *think* @willkg might have that already from when I implemented that and he r?'d.)

Anyhow.
r?
This should be an ngettext and not a gettext since "contributions" might be singular, plural or whatever.