Fix2582 corporate workshop costs

[maneesha]

Fixes #2582 by removing any direct references to workshop cost, and only referencing The Carpentries website.

[maneesha]

I will come back to this to fix those linting errors. @pbanaszkiewicz If I do so, would you recommend running additional migrations or deleting the migrations in this PR and replacing them?

[pbanaszkiewicz]

@maneesha There's a good chance that if you fix this kind of error in the both models.py files, you won't need a new migration.