CLC-1844 Drop management of individual Canvas OAuth2 tokens

https://jira.media.berkeley.edu/jira/browse/CLC-1844

CalCentral's Canvas integration now covers anyone with an account, the same as with bSpace and campus systems.

One note when testing: Canvas supports different levels of "administrator", and naturally enough an account at a lower level can't "act_as" an account at a higher level. If you find that you don't get results for a particular couple of users (notably myself & Jon), the application server might be configured with an underpowered admin account.

This commit also includes renaming the user-specific "CanvasCoursesProxy" to "CanvasUserCoursesProxy" just as a convenience to my own sanity as we start doing more administrative work with Canvas Course sites. If that seems off to you, let me know & I'll repackage the PR.