Update Mzalendo design

Fixes https://github.com/mysociety/pombola/issues/1359

### Header and footer:

- [x] Remove survey header (if it's not already been removed by https://github.com/mysociety/pombola/pull/1368)
- [x] Simpler, flat green header; less fussy search box
- [x] Wide navbar, no gradients
- [x] Dropdown arrows on relavant nav items?
- [x] Dark grey footer to match navbar and contrast more with page content


### Clean up homepage:
- <del>New hero image?? (small?)</del>
- [x] Prominent links to Search, Hansard (and Rights?)
- [x] Quote from Kenyan constitution
- [x] Large, centred mission statement below hero
- [x] Larger links to blog posts, more whitespace
- [x] Narrower sidebar containing <del>tag cloud</del>, featured person and Twitter feed

### Redesign person page:
- [x] Grey page background, to make content blocks stand out
- [x] More friendly 'polaroid' style profile pic
- [x] Less abrasive party / county links under profile pic
- [x] Distinct sections for Experience, Appearances, Contact Details etc.
- [x] Move Share / Correct / Comment buttons alongside title
- [x] Generally better typography
- [x] Work out how to make it scale down to small screens
- [x] **Change from mockups:** Put Disqus comments into their own section
- [x] **Change from mockups:** No all-caps headings anywhere
- [x] **Change from mockups:** Put politician's title and Date of Birth below their name
- [x] Jessica also suggested putting the contact details below the picture, like [People's Assembly](http://www.pa.org.za/person/agnes-noluthando-daphne-qikani/)

### Refactoring:
- [x] Pull Mzalendo styles out into separate theme as far as possible