Add demo election content

[stephen-cox]

Fixes #28

[markconroy]

@stephen-cox this looks great, but is also creating duplicate taxonomy terms for anyone who already has the localgov_elections_uk_parties module installed.

Do you think we could get this to use the UK Parties module for the taxonomy terms instead of generating new ones? Or, failing that, delete the generated nodes/terms when uninstalling this module?

[finnlewis]

note: want to check if we can remove the content when uninstalling the module

[finnlewis]

@markconroy I've added '(demo content' to the term labels for now.

Looks like there is a drush command in the pipeline to remove content on uninstall, see https://www.drupal.org/project/default_content/issues/3282547

For now there is not much content, so I think it would be easy enough to remove the demo content if desired.

[finnlewis]

Makes it so much easier to test, thanks @stephen-cox !

[finnlewis]

Not sure why the tests are getting a 500 server error here, they all still pass locally.