Add professional info fields to the sign up form

[Refs #72499862]
Add Profession model

[Refs #72499862]

Generated Profession model, migrated the database.
Add user-profession relationship

[Refs #72499862]
Added many-to-many user/profession relationship.
Created join table, migrated db.
Update welcome.html.slim

[Refs #72499862]

Changed the layout a bit, added input for professions.
Update users_controller

[Refs #72499862]

Made profession ids a permitted field.