Front-end polish work

Fixes https://github.com/mysociety/writetothem/issues/127

## General
   * Add accessibility features
      * [ ] ARIA roles
      * [ ] Check hover/focus declarations are present
   * [ ] MS header hover is weird
   * [ ] MS header a bit small on tablet layout
   * [ ] Should help text be source sans?
   * [ ] Mobile typography generally needs some work - needs to be stronger
   * [ ] Make the favicon background transparent
   * [ ] Too much padding around the tablet layout
   * [ ] #134 Remove outer box on layout
   * [ ] Double check background image sizes on retina displays

## Homepage
   * [ ] Position of text in image is not quite right
   * [ ] Width of postcode form is too narrow on tablet
   * [ ] Position of logo and MS tab on mobile version is a bit muddled

## Select representative page
   * [ ] Indentation is weird on mobile
   * [ ] Have you spotted a mistake? Could be worded - ‘Correct a mistake in this list’
   * [ ] Add hover styles from design to MP names
   * [ ] On tablet sized the typography drops too much in size
   * [ ] Should member of parliament just be 'MP'? 

## Write a message page
   * [ ] Padding around the message input box
   * [ ] Vertical spacing between ‘Ready?’ and the blurb
   * [ ] Links in the ‘instructions’ pane need hover styles
   * [ ] Tablet layout is broken - LH column is too narrow, writing area should go full width

## Nice to have: Performance improvements
* [ ] Sprite up css images
* [ ] Move retina image declarations to a separate stylesheet
* [ ] Base64 anything that seems sensible
* [ ] Recompress photographs etc