organisation: add slug, blurb, description to theme model

Slug is needed to make dynamic theme pages possible as part of a
cms app. Slug is automatically populated in the admin.

Blurb is the text that will be in the blurb of dynamic theme
pages.

Description is the main text on a theme page.
Merge pull request #122 from okfn/theme-pages

organisation: add slug, teaser and description to theme model