Accessibility improvements

[smartspot2]

Currently, scheduler does not have any accessibility information built-in; see https://accessibility.18f.gov for guidance on accessibility.

Some major points:

• Contrast between colors is horrible (reference, checker)
• Buttons, links, and other interactions are not keyboard-focusable (reference)
• Page titles are not updated between subpages (reference)
• aria-* attributes are not specified

The site should be looked over, and various accessibility attributes should be added. This is also a good opportunity to add other kinds of descriptive text to all interactive elements. Colors should also be reworked to increase contrast.