adds a dedicated card view mode

[markconroy]

Closes #57

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Thanks to Big Blue Door for sponsoring my time to work on this.

[ekes]

Woo... popped in here to see if there was anything backend-ish to check and there ain't so much. It adds to existing sites, but uses the localgov_ name space so that should be safe. So pass on that, but while I'm already here, questions:

I know card display modes are pretty common, is the plan to use it somewhere in the default localgov install? I'd assume that would be the reason to want to add it to core? At the moment it's going to have all the default fields because there's no settings for content types? Is there a plan for where this is going?

[andybroomfield]

Does this actully enable it on anything though? Is it planned to be used anywhere?

[markconroy]

Yes, we should start using it for the featured news at the top of news listings for example.

We could eventually start shipping teaser and card view modes for each of our content types (that can be then used in views or in entity reference fields).

I already have a follow-up issue created in LocalGov Base to add a card template.

Based on the work for elections recently, I can also imagine a "timeline" view mode in our not-too-distant future.

[markconroy]

@andybroomfield @ekes are you okay for us to mark this as 'approved'?

@msayoung any thoughts?