Add username_key as an LTI 1.1 Authenticator configurable

[jgwerner]

• Adds the LTI11Authenticator.username_key to set the username returned by the authenticator to something other than custom_canvas_user_id or user_id if specified
• Adds basic tests for LTI11AuthenticateHandler class
• Updates Readme

Continuation of #14

Closes #30

cc/ @BenGig

[yuvipanda]

Awesome! Couple minor things, happy to merge afterwards

[yuvipanda]

What do you think of making a release after this gets merged?

[jgwerner]

@yuvipanda creating a new release sounds good to me! I'll update this PR based on your comments, thanks for the review.

[consideRatio]

@jgwerner wow amazing work!!! I appreciate how thorough you are!

Excellent documentation, excellent testing! ❤️ 🎉

Assuming @yuvipanda comments are addressed, this LGTM so I'll press approve at this point!

[jgwerner]

@yuvipanda I updated the PR with the following logic:

• Log a deprecation warning if the username_key = "custom_canvas_user_id"
• Check if the username_key is in the launch request
• If the username_key is not in the launch request raise an HTTP error
• If the username is empty or None, fallback to the user_id value
• If the username is still empty raise an HTTP error

There were also some duplicate validator tests in the authentication tests file so removed them. Perhaps we should have refactored tests in another PR, but wanted to make sure the flow above had passing tests without duplicates.

[yuvipanda]

@jgwerner thank you for your patience :) This lgtm for the most part - I left some documentation change suggestions, what do you think? Happy to merge this after.

[jgwerner]

@yuvipanda I update the PR based on your suggestions, thanks for the documentation tips 👍

[yuvipanda]

Thank you, @jgwerner!