Improvements to "report a problem" page

This closes https://github.com/mysociety/fixmystreet/issues/381.

![screen shot 2014-07-22 at 16 01 52](https://cloud.githubusercontent.com/assets/739624/3659827/48a1f612-11b1-11e4-9860-bef9cd67bb74.png)

A number of changes to the FixMyStreet cobrand, aimed at improving the "report an issue" page:
* White "page" set flush to sides of window, to save space and remove feeling of a big white box getting in the way.
* "Tips" sidebar removed to make more space for map.
* Form reworded to include most important bits from the old "tips" sidebar, and make them more noticeable.
* Nav bar cleaned up – logo is no longer flush against left hand side, and nav menu is no longer stuck in the middle of the screen.

The first of these changes was made to the base styles, but the rest are limited to the FMS cobrand because I couldn't get my head around the HTML template situation.