[POC] Allow custom app

[gautamsi]

Allow using custom _app template for admin ui

@flexdinesh this how we can do natively in keystone instead of my file watcher workaround.

I have made changes to examples/basic template to use custom theme. notice button color changes when running the modified example.

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Name	Status	Preview	Updated
keystone-next-docs	✅ Ready (Inspect)	Visit Preview	Oct 19, 2022 at 9:43AM (UTC)

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[gautamsi]

let me know if this approach is acceptable then I will cleanup the PR and write documentation

[deibeljc]

@flexdinesh I am currently really looking for this capability. Are there still plans/work being done to replicate this behavior in some manner? Or is finishing out this PR the only way to get this functionality in the short term? If so I could see if I can address the comments you have made in order to get this merged!