adds CSS to allow tables to scroll on small screens

[markconroy]

Closes #579

What does this change?

1. Sets .layout__region and .field--table to max-width: 100%, so they don't overflow with any tables (or other elements)
2. Sets the .tablefield-wrapper to use overflow-x: auto so if it's too wide, it will have a horizonal scroll.

How to test

1. Visit /test-subsite-demo-all-components/example-page-tables-and-tabs on a small screen and check that only the table has horizontal scroll, not the whole page.

---

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