Accessibility: Create ARIA landmarks

[cjcenizal]

ARIA landmarks (or the equivalent HTML5 sections) are used to identify some significant page areas. Every page should have a <main> element (or role="main"). ARIA regions with descriptive labels should also be used for major sections within the main content (see https://www.w3.org/TR/wai-aria-practices-1.1/#aria_lh_region).

We can apply these ideas by doing the following:

1. Update the "main content area" into which apps are rendered to be a <main> element instead of a <div>.
2. Use the <section> element to identify major sections of each app, e.g. the Discover field chooser, the Discover chart, and the Discover table.
3. Use role="search" to identify instances of the query bar in the LocalNav and instances of SearchInputs used to filter lists and tables.

[timroes]

This should be done by now. If there are any places left this isn't done we should open a separate issue for that specific place, UI component, etc.