Move mySociety logos to the asset pipeline
Remove images that are no longer used
Move the custom stylesheet to be used in the asset pipeline
Remove inclusion of old stylesheet.
Generalise asset path prepend for all standard types.
Generate custom.css from SCSS & use the sass-rails image-url helper
Add back the old mysociety logo images - not sure why these were dropped
Fix the serving of two images
Make sure images are referenced with image_tag

This is so that they will be correctly served by the asset
pipeline.
Merge remote-tracking branch 'origin/switch-to-asset-pipeline' into develop
No need to symlink the theme's public directory.

Now that we're using the asset pipeline, that takes care of making
assets available.
No need to unsymlink

Superceded by asset pipeline
Merge branch 'develop'

Conflicts:
    assets/stylesheets/custom.scss
Merge branch 'master' of ssh://git.mysociety.org/data/git/public/whatdotheyknow-theme