TPAC: Address severe accessibility issues

Using the Chrome Accessibility Dev Tools extension to run accessibility
audits against the TPAC turned up some "severe" issues, such as input
widgets lacking labels. In most cases the text for the labels was
present, but it needed to be associated with a <label> element. In some
other cases, it was easier to use the aria-label attribute to give the
widget an accessible label. Finally, we can use the placeholder
attribute where warranted as an accessible label.

These changes address the first set of accessibility issues I came
across via the audits and could address quickly.

Signed-off-by: Dan Scott <dscott@laurentian.ca>
Signed-off-by: Mike Rylander <mrylander@gmail.com>