Add a new format for Campaigns

This is a new format for campaign pages, which are moving inside the publishing platform.

A campaign is similar to a regular Answer, but has a few additional fields:

- `organisation_formatted_name`: the name of the organisation running the campaign, formatted with line breaks as per its logo
- `organisation_url`
- `organisation_crest`: the type of crest to use in the logo (from a predetermined list)
- `organisation_brand_colour`: the department brand colour to use in the logo (also from a predetermined list)

A campaign also has three assets, `small_image`, `medium_image` and `large_image`.