A11y Fixes

[chelseasimek]

• Aria Command Name for all paper-icon and paper-tab components
• Aria Required Parent for all paper-tab instances
• Color contrast on yellow buttons w/ #fff text
• Link name on social icon links: need to add aria rules since these do not have link content in them
• Nested-interactive elements for navigation items - these need either focusable descendants or need to be changed to a different semantic HTML type
• All images (esp in gallery) need alt text.

[chelseasimek]

All but the nested interactive elements and alt text has been fixed, comments below, to be fix in separate tickets.

1. The paper-tab elements are role=tab so they are "interactive" elements. They expect a flag of link to look for an a nested inside of the tab. This breaks the nested interactive accessibility rule (both tab and a are interactive and will confuse a screenreader), and it's making the entire navigation at the top not keyboard navigable.
2. The gallery and speaker cards don't have alt tags because they are plastic-image elements and that element doesn't pass down the alt property to when the semantic img tag is eventually made in the shadow dom.