Add SimpleForm initializer
Add a new sign-up form

Adds options for the user to select whether they're an individual,
teacher, or student, and adjusts the fields which have to be filled in
depending on the choice made.

Removes the normal UI to remove distractions, and adds texts clarifying
the meaning of "teacher's e-mail" for students.

Closes #1777. Closes #1778.
Added Dutch translations
Merge pull request #1782 from quintel/new-signup

New signup form