Added protection on the BE to ensure attempts to launch_consensus multiple times. See description for additional notes.

Similar to finalizing SRP configuration, we needed some added protection in the BE to prevent an attempt to launch consensus multiple times. Error for this situation is now surfaced to the FE and there is an additional test on the service.

In the FE, the button is not disabled after successfully launching consensus as we need to work out how to detect that PPR proposal reviews were created and updated in the cache. This is captured as a note in STT-1581.