Allow user info to not require username, refactor affected functionalities

[nickjhall]

Description

This change introduces a new property loginInfo.usernameRequired in the config file, which is set to true by default. If it is true, there is no change to the website. If it is set to false, the signup, login, and account update templates no longer include a username field (with email replacing it on the login page), and the user model does not include the username property. The authentication API and model have been changed slightly to function with either username or email.

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.