Implement editing model servers

[markwinter]

Currently it is not possible to edit existing model servers from the web app.

This PR adds a new edit function that lets the users change the existing YAML.
It uses the k8s replace_namespaced_custom_object in the backend.

This will resolve #2 because the user can add the canaryTrafficPercent and change the storageUri

@kimwnasptd Can you review this please?

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[markwinter]

/retest

[juliusvonkohout]

@markwinter Please rebase to master

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[juliusvonkohout]

Alright then you have to fix the broken tests.

[juliusvonkohout]

Can you rebase to master and fix the tests?