Expanded tests for user routes and other tweaks

[webbpinner]

I'm just learning about FastAPI and this project was incredibly useful!

PR Summary:

• Expanded tests of user routes to increase coverage
• Moved "/user/me" route to "/profile"
• Added check to avoid 500 error when adding a username already in the db
• Added refresh_token mechanism and associated tests

Found the same bugs listed on a previous PR. While expanding the tests for the user route I found the ordering bug listed in another PR. I started by re-ordering the functions but ultimately decided the "/user/me" route was too risky (for me) i.e. a user is created with the username of "me"... thus I elected to move the route from /user/me to /profile.

Added a JWT refresh_token mechanism that I'm going to need for my next project. This includes the new /refesh_token POST route for updating the the access_token. The access_token now includes a boolean "fresh" value in the JWT to indicated if it was issued based on a /token request or by the refresh_token request. Added a new depends (AuthenticatedFreshUser) to require that the user must have a "fresh" access_token or be a superuser.

[webbpinner]

just found the instructions for contributing... will re-submit as instructed.