Update User schema

[vpchung]

This PR addresses #120,. specifically:

• removed properties: role
• added properties:id, description, verified, profileImageUrl
• related to above, added a UserId schema to components/parameters/
• updated the paths for User so that a user can either be retrieved by userId or username

For convenience, the updated User schema now looks like this:

property	type	description
id	integer	unique ID of the user
username	string	unique username of the user
description	string	short bio about the user
verified	boolean	indication of the user's verification on ROCC
profileImageUrl	string	URL to the user's profile image
firstName	string	user's first name (extended from Person schema)
lastName	string	user's last name (extended from Person schema)
email	string	user's email address (extended from Person schema)
organizationIds	array of strings	organizations the person belongs to (extended from Person schema)

and the updated path is:

.../users/{user}

where {user} can either be UserId or Username.

[tschaffter]

@vpchung I made a few cosmetic changes to the User schemas - changes inspired from Schemas.org and GitHub, otherwise it's good to go. I'll use this User schemas to update the API service. On the web app side, the User schemas will allow us to start working on the user profile page and add features like favorite challenges.