Add common utility functions for LTI 1.1 and LTI 1.3

[jgwerner]

• Adds common functions to normalize strings, fetch usernames, etc to keep things DRY
• Updates utils tests

Closes #42

[jgwerner]

@yuvipanda most of your comments from #44 about escapism will be are dealt with here and then we can rebase update the authenticator, tests, and rebase #44 before merging.

[yuvipanda]

Thanks, @jgwerner!