Edit Knative domain post-deployment
### Problem to solve
Once I deploy Knative to my k8s cluster, it is not possible to edit the domain.
### Target audience
developers, operators
Existing personas are: (copy relevant personas out of this comment, and delete any persona that does not apply)
- Sasha, Software Developer, https://design.gitlab.com/research/personas#persona-sasha
- Devon, DevOps Engineer, https://design.gitlab.com/research/personas#persona-devon
### Further details
<!--- Include use cases, benefits, and/or goals (contributes to our vision?) -->
### Proposal
We should be able to update the domain after deployment. Under the hood this is just a `kubectl patch` of the configmap. Knative updates the service hostname automatically when the config map is changed.

## Solution
Add a `Save changes` button. We can simplify the form a bit in the process:

If Knative has not yet been installed, it would look the same as today.
### What does success look like, and how can we measure that?
Update a Knative field on your k8s clusters many times without a problem and without upgrading your knative version.
### Links / references
issue