universal footer: redo markup + styles with flexbox etc.

[alisonjo315]

Follow-up to #48

• NOTE: May end up transferring this issue to cwd_project, if/when Templates: site footer #46 is done.

There was a recommendation to redo the footer markup + styles, using flexbox + justify-content treatment, as seen on OADI:
https://test-oadi.pantheonsite.io/

This change was started in #49, but then it seemed clear it should be its own issue/discussion. Prior start to implementing this change:
https://github.com/CU-CommunityApps/cwd_base/pull/49/files