* 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)
Banner sections can be saved (one banner can appear in several sections)
If the hex color is changed in the textfield, the color of the color picker changes.
