Improve accessibility of add/edit event form required fields

[carrythebanner]

On the add/edit event form:

• Required input fields now use the required attribute.
• The asterisk in the required field's label is now a visual only cue (it will be skipped by screen readers).
• The visible "@" and "to" labels for the date and time fields are skipped by screen readers, and more explicit, non-visible labels have been added for screen readers.

For more background on these, see:

• Improve required field indicators for screen reader users on "Add an Event" form calagator.org#30
• Add accessible labels for start and end datetime fields on "Add an Event" form calagator.org#31

[aeschright]

Thanks @carrythebanner -- this looks great! Can you fix up the merge conflict so we can pull this in?