LP1887869 WIP Revised form labels for item edits

LP1887869 WIP Revised form labels for item edits

Revises the accessibility work previously done on the item edits and
item templates tabs to simplify the <label> strategy and avoid
potentially empty aria-labelledby attributes (which would override any
other valid labels).

Also changes <a> to <button> for click events, to allow keyboard access.

Signed-off-by: Stephanie Leary <stephanie.leary@equinoxoli.org>