Improve a11y of address search form on slide 2

[Chris Henrick]

The address search form's accessibility could be improved via the following:

• applying aria-required="true" to the <input> element
• applying aria-labelledby to the <input> element to associate it with error text
• possibly using aria-live="assertive" around the error validation text
• applying aria-invalid="true/false" to the <input> when it is in an invalid state
• apply focus to the input when the user submits the form when there is an error

Some of these depend on which form validation strategy is being used. See this write up in Smashing Magazine for more.

[Chris Henrick]

Implemented in #128