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.

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

[msayoung]

@markconroy - this isn't working on Chrome and Edge, though is fine on FF.

Not sure how but in DevTools the css is

.tablefield-wrapper {
    overflow-inline: auto;
}

which isn't a thing.

[finnlewis]

@markconroy @msayoung : This does work for me on Chrome and Firefox, Mac.

[finnlewis]

I think this works, certainly an improvement, but worth testing in other browsers perhaps?

[finnlewis]

Discussing in Merge Tuesday, we're going to merge and come back to fix things for Chrome @msayoung if neeeded.