Add email disclaimer to submission form

[litnerdy]

Currently, it's mandatory to enter your email address when you submit a student debt story. We don't say this on the site, but the only reason we need the email is as an anti-spam/troll measure. We don't actually use the email for marketing or any other privacy-violating reason, so we should make this clear up front.

Impact: This may help people who are hesitant about giving their email out to a random website, and unblock/unlock more submissions.

To do:

• Draft copy for a hover over over something similar that explains why we're collecting the email address
• Get feedback on the copy
• Choose a design/layout for the content
• Publish new content