Add `shared_with_group:` and `shared_with_user:` filters to /workspaces endpoint

[brettkolodny]

Implement the following filters to the /workspaces [get] endpoint:

• shared_with_user
• shared_with_user_id
• shared_with_group
• shared_with_group_id

An example for comparison is how we handle the owner filter. There’s also an owner_id filter which we set (and then clear the owner filter) before actually starting the db query.

Note

The above is taken and restated from the RFC