Add cookie consent banner to base.mako [PLAT-904]

[erinspace]

Places a banner on the bottom of all pages on cos.io with links to the privacy policy and cookie information.

When clicking Accept, sets a cookie to remove the banner that expires in 30 days.

Also apparently my editor removed a lot of trailing spaces in style.css - this wasn't intentional but probably for the best, lemmie know and I can put them back.

Ticket

https://openscience.atlassian.net/browse/PLAT-904

[erinspace]

Note -- banner is now same color as OSF's banner

[erinspace]

Link text is now white and underlined to match https://github.com/CenterForOpenScience/ember-osf/pull/397#discussion_r201705581 and https://github.com/CenterForOpenScience/osf.io/pull/8524

[erinspace]

link colors upadted