Split agent templates into multiple pages

[timja]

Matches jenkinsci/kubernetes-plugin#1443

Simplifies editing of agent templates so that you know exactly which one you are editing.
It will also improve the performance of the cloud configuration page as well.

Cloud configuration page

Agent template page

New agent page

Delete agent template

Edit agent template

Testing done

I was able to create new agent templates and update existing ones
I was able to validate both basic and advanced templates
Validating basic templates requires this PR to core: jenkinsci/jenkins#8832

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