Simple theming

[ross]

Needs re-testing after the ODS re-theme. ====

Simple image upload, text changes and css updates for a single instance. All done on a single configuration/admin page

See ​http://ckan.okfnpad.org/simple-themes for user stories

[x] Initial version will only take an image_url rather than allowing an upload.

[x] New page with form for title, tagline, image loader etc.

[x] Allow the user to paste in values for the CSS, do we really want a colour swatch?

[x] Generate local CSS override which should *somehow* override colors in the original CSS.

[x] Allow user to provide text for an about page.

[ross]

Will eventually want to allow customisation of different CSS but designs are currently under review/change.