UI: make somewhat skinnable / themable

[alexec]

Summary

Allows users to skin the UI with new colours, their own loge, but keep Argo bits.

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Message from the maintainers:

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[simster7]

Proactive users should be able to do so by forking the repo and making simple CSS changes... is there a good incentive why we should make this a paved road?

[alexec]

Yes. People could do that, but I've not seen any examples. I have users write their own bespoke own UI.

Instead, I'd like to make it easy to use our current UI for that.

Part of that is generalising and making pluggable/skinnable the UI. Generalizing - it exposes any Kubernetes resources based on configuration.Pluggable - add their own routes, UI components, etc. Skinnable - use their own colors etc.

Once fully generalised, workflows, events, and dataflow just become plugins.

[krrrr38]

In my cases, it's enoght to change banner colour by environments.
It may be possible by argocd this feature https://argo-cd.readthedocs.io/en/stable/operator-manual/custom-styles/.

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oops, this feature is related to #1451

[agilgur5]

See also #10825 for custom themes