Accessibility audit fixes for Combobox, DataTable, Expression, Map, and Dropdown

[kevinparkerson]

Fixes #2358

• Combobox: Headers/footers need to read properly and most recent item should be immediate previous tab index after selection
• DataTable: aria-live needs to be removed, checkbox names need to be unique
• Expression: focus needs to move to first input upon adding new condition or group
• Map: focus needs to move to map when selecting a location
• Menu Dropdown: styling needs to be corrected for header text orientation

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REVIEWER checklist (do not remove)

• TravisCI tests pass. This includes linting, Mocha, Jest, Storyshots, and components/component-docs.json tests.
• Tests have been added for new props to prevent regressions in the future. See readme.
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Required only if there are markup / UX changes

• Add year-first date and commit SHA to last-slds-markup-review in package.json and push.
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[interactivellama]

I'm going to go ahead and merge this. Map: focus needs to move to map when selecting a location should be working based on the code, but I'm not hearing the iframe's title read in VoiceOver (for instance).

[interactivellama]

Great solution!

[kevinparkerson]

@interactivellama VoiceOver is behaving really inconsistently; the first time you load the page, tabbing to it will put a box around it and it reads "frame 1, Salesforce Locations In United States, group". Shift tab and then tab back to it, it reads "group, Salesforce Locations In United States, group" with no box around it. Additionally both times there's a lag period where it says "Chrome busy" before it reads or highlights anything. Additionally, the iframe never shows a visual focus indicator without some sort of screen reader being activated. There are two solutions I can see for this:

1.) inject styling that makes the iframe have a highlight when focused like most normal elements do
2.) change the focusable element to the .slds-map div. We'd also need to move the title prop to that element so it'd read correctly. This diverges from SLDS markup a bit though

Let me know what you think, I can try to address this in a separate PR