Can't create an execution environment with the same name as a deleted EE.

Current behavior: If an execution environment has been created and named "test", and then deleted, any attempts to create an EE with the same name "test" will fail with a 400 error because "This field [name] must be unique".*

Expected behavior: the names of previously deleted EEs shouldn't affect anything about what EEs I can create after their deletion.

Steps to reproduce:

• create an execution environment, e.g. named "test".
• delete it.
• attempt to create another EE with the same name (with the network tab in your browser dev tools open.)
• observe the 400 response code and the error in the response body.