Add ability to set a user type and assign institutions

When creating or editing a User, add the ability to set the user as either an:

• admin (ability to manage everything about everything)
• institutional (ability to manage everything about 1..n Institutions)
• helpdesk (ability to view Institution information and manage Contact information)

add helper methods to the user class to check for roles, e.g. admin? institutional? helpdesk?

add a required string field to User that stores this value

add a optional m2m relation between User and Institution, and corresponding User form elements

add a <select> to the User form to set this value

in the migration, set existing users to admin. also be sure to update user created in seeds file.