Improve sign-up form

Currently, the sign-up form for supervisors does not indicate that one must use their "institutional" e-mail address. I propose we modify it slightly to indicate this as well as include an URL to the reasoning behind this.

For example:

• Instead of saying "you hold a position in a research laboratory", we should say "you hold a permanent position in a research laboratory".
• Instead of saying "E-mail" we should say "Institutional E-mail".
• We should add a link to: https://www.idiap.ch/software/beat/docs/beat/docs/stable/beat.web/doc/user/faq.html#what-is-a-supervisor
• In that link, we should specify that e-mail should be institutional.