Feature: Support multiple "profiles"

[twavv]

Currently we embed all the information about a codex in the codex.toml file (including which course/codex category it belongs to).

One idea might to be have multiple profiles:

# Upload the same codex to two different courses
pbauthor codex upload --profile stats123-f21 .
pbauthor codex upload --profile stats456-f21 .

This doesn't perfectly answer which codex category to choose, but we could build in logic that says "if was already uploaded to this course, find the category it was uploaded to and replace that one, otherwise prompt which category to upload to":

# On first upload
$ pbauthor codex upload --profile stats123-f21 .
Uploading "Intro to R" to course "Statistics for Undergrads"...
Choose category:
  [*] Lecture
  [ ] Homework

$ pbauthor codex upload --profile stats123-f21 .
Uploading "Intro to R" to course "Statistics for Undergrads"...
Replacing codex in category "Lecture"... [y/n/c]

We'd still have to store some kind of deduplication key in the codex.toml file.