Elastic UI Modals

[aphelionz]

Subset of tickets in #614 that deal with the "Modals" category of EUI, anything that falls on or under this URL: https://elastic.github.io/eui/#/guidelines/modals

• First modal example does not adequately focus the primary modal button First modal example does not adequately focus the primary modal button #700
• Dialogs Need to Signal Context Switch to Users Upon Opening Dialogs Need to Signal Context Switch to Users Upon Opening #712

[rayafratkina]

An A-level issue in ML depends on this, so added a WCAG A label.

[rayafratkina]

@snide the issues this ticket links to are both marked as documentation, however Walter believes the ML issue will be fixed when these are addressed. I think this means those issues are not just documentation-related and need to be fixed. Correct?

[walterra]

Here's a related EUI issue about the accessibility of modals: #502

[chandlerprall]

EuiModal accessibility is now being tracked by #3889