Page: Volunteer Applications - Submitting and error checking

[some-molasses]

When the volunteer application form is submitted, we'll need to be able to validate the form and let the user know their application was successful.

Mostly blocked by #159, which handles the form frontend.

Acceptance criteria:

• The volunteer application form can be submitted to the server. It doesn't need to be able to do anything with the form yet, that can be handled in a PR for Page: Volunteer Applications - Handling valid forms #161.
• The server has error checking to ensure bad values aren't submitted:
  • All email addresses are valid
  • The user can only apply to roles that exist (they cannot submit arbitrary strings)
  • A person cannot submit the same application twice within the same term
• When the user submits the application form, they are redirected to a page thanking them for their application:
  • Example text from the current site's flow: "Thanks {inputs.names.middle_name}  for your application. We will get in touch with you shortly. To submit another form please go back to the volunteer page."