[Core API] Create discussions

Discussions may now be created with the POST /conversations endpoint.

• A discussion may be created in an inbox (assuming that the API token/OAuth client has access), or with a specific list of teammates.
• The initial comment in a discussion may be authored on behalf of a teammate or by the API token/OAuth client itself (this behavior is similar to adding comments to existing conversations).

Discussions and related activities may be retrieved from a few endpoints, depending on the "audience" of the discussion. Each combination of audience & endpoint is marked below with ✅ (supported) or ❌ (not supported).

Note: For any retrieval method to succeed, the API token/OAuth client must have access to the relevant resources (e.g. inbox, teammate resources, etc.).