Add CSP for Returning Teachers workshop type

[islemaster]

Introduces a new workshop type "CS Principles Workshop for Returning Teachers" as the first step in implementing this spec.

Also (temporarily) removes the ability to create a workshop of this type by hiding it from the workshop dashboard dropdown.

Merging this first so that Ben and I can split up the remaining work of making this workshop type function as expected.

Testing story

Checked with a local build that the new constant exists but is omitted from the dropdown.

Reviewer Checklist:

• Tests provide adequate coverage
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[bencodeorg]

LGTM -- just did a little double checking and found this reference that I think will evaluate to true for the new workshop type (I think can deal with it later, added to the list of to-dos in the doc we were working in).