buttons should be readable by screen readers

[techieshark]

@reekypete pointed out that button controls could have labels which are more self contained (so screen reader users wouldn't have to have surrounding context to make use of the buttons).

A couple thoughts:

• now that we're using more semantically correct <button>s rather than <a>s, we'll need something other than title attribute to make this work.
• I came across documentation about what to do in this case while doing research; I'll have to dig that up. We might use <span class="sr-only"> (screen reader only) to add that context into the button text, or use ARIA labelling.
• this issue may relate to all buttons we use, but we should at least start with the primary / secondary school buttons at the top.

[techieshark]

example using <span class="sr-only">Find a school for a </span>: