A11y: Introduce Alternative text to the Message Search control

[daniellekirkwood]

In the Room, users can click the search icon to search messages. There's no alternative text here and therefore is not usable for screen readers.

Please ask @daniellekirkwood about timesheeting

[t3chguy]

The header button to reach this state has a label - this issue is about the search action button which can also be triggered by Enter in the text field

The X button is also unlabelled