Revamp Volunteer form; move old fields to Register

[kfranqueiro]

This PR makes the following changes:

• Moves existing fields from volunteer form into registration form
  • This includes adding validation for those fields and relocating Do you mean email #87
• Adds an extra error region to the registration form near the captcha field, since the form is now likely to span more than a single viewport height
• Implements new fields (checkbox groups) for volunteer form
• Only displays volunteer form when signed in; otherwise, links to sign in page

New breaks intentionally introduced in this PR:

• The checkbox groups in the volunteer form are not described correctly:
  • One uses role="group" with aria-labelledby, but pointing to the wrong element from a different group
  • One uses role="group" with no aria-labelledby
  • One does not set role at all
  • Discussion item: Only the first of the three above cases would explicitly fail WCAG 2 (1.3.1 Info and Relationships); should WCAG 3 more clearly cause the remaining two cases to fail?
• The "other" textarea only displays when "other" is checked, with no announcement or label
  • Discussion item: what would be the optimal correct behavior for this?

(These breaks are not currently documented within this PR, because I intend on documenting them in #89 instead, and doing so here would create additional work including merge conflicts)

[fstrr]

The "other" textarea only displays when "other" is checked

For me, the textarea is visible from the start when the page loads.

[kfranqueiro]

For me, the textarea is visible from the start when the page loads.

Big oops, thanks for catching. This broke when I switched all of the labels to use for instead of containing the inputs. Should be fixed now.

[fstrr]

The form times out pretty quickly. If you were going to complete the checkboxes and then write a comment in the textarea, you'd likely not be able to before the timeout happens. We should either lengthen it by a 2-3 minutes or discuss whether we want the timeout on this form as well as the register one.

[kfranqueiro]

The form times out pretty quickly. If you were going to complete the checkboxes and then write a comment in the textarea, you'd likely not be able to before the timeout happens. We should either lengthen it by a 2-3 minutes or discuss whether we want the timeout on this form as well as the register one.

If we remove the timeout from the volunteer form, we will still have failures in the gift shop checkout flow's shipping and payment pages. If that's sufficient I'm not against removing the timeout from this page entirely; I don't remember what our motivation was for adding it here.

@rachaelbradley, thoughts?