[D8]Implement Footer Variations

[iris-i]

There are 3 variations of site footers in the playbook, Small, medium & large. How do we offer these choices to the users?
https://playbook.cio.gov/designstandards/footers/

From #15

[iris-i]

I wonder if this can be done using the configuration file?

[fureigh]

Great question! We can offer these settings in the .info.yml file, yes?

[fureigh]

Echoing #48 (comment): "Any styled footer" is the minimum viable element here; feel free to break out another ticket for config if you'd like.

For ease of tracking:

• small implemented
• medium implemented
• large implemented
• config ability implemented

[brockfanning]

In #67 I handled this using a normal theme settings form.