Add citizens assemblies

[ajparsons]

This PR adds citizens assemblies on climate to the data displayed in CAPE.

Importing from https://github.com/mysociety/citizen-assembly-data, it adds:

• Citizens assembly reports as a new document type that is displayed.
• A new council card for councils with an assembly to show just that data type and some explanatory text.
• A new filter to the list view that lets you search for councils that have had a climate assembly.
• Amended the search to allow searching by document type (and so limit a search just to assembly documents).
• A new top-level page to explain assemblies, and link to the list and search views.

Management commands to run:

script/manage import_citizens_assemblies
script/manage rebuild_index --noinput
script/manage import_related_searches --all

This gets in the new data, rebuilds the index, and redoes the related search with a slight tightening of strictness to improve relevance.

[zarino]

PS @ajparsons I’d be inclined to squash all of these commits into one, given they’re all little bits of the one page:

• Add new assembly top level page
• Add extra card for climate assemblies
• Add download link to citizens assembly data
• Add details of Scotland assembly to blurb
• [CAPE] Styling for climate assemblies homepage