Using `sanitize` we make sure the `href` attribute does not execute any
dangerous code. The possibility of a banner pointing to a dangerous URL
was very reduced, though, since only administrators can edit this
attribute.
* Delete all things related to banner images and styles (in code)
* Add a new test to check that the banner is showing correctly
* Update the specs accordingly to match the changes
Update dev_seed to set a random background_color and a font_color for banners (and remove everything about image and style)
Add a rake task to migrate the banner style to backgroun_color and font_color (so that the banners have the same colors than before)
