KE: Replace address search box with county-level dropdown

[jacksonj04]

Replaces the search box on the home page with a dropdown of counties.

[mhl]

A couple of quick notes on this since I'm away for a few days:

• We still have to decide if this is the right thing to do - I would still prefer to do a quick series of prototypes, as we discussed at the end of last week, even on a static page or on jsfiddle with different options for selecting areas from a drop-down, including variants which included constituencies or even wards, which we can then get people from Mzalendo to try.

• This should really have a test or two in elections/kenya/tests.py (the test failures at the moment should be fixed by rebasing over master now)