Registry UI APIs

[haoming29]

Closes #264

• Please refer to Swagger UI / Swagger documentation for available APIs
• This PR didn't include support for custom registration field to scale down the scope (it's already ~2000 lines of change!), which will be a separate PR
• Other follow-up tickets will be added to improve test coverage and improve the robustness
• Note: This PR breaks the Web UI for registry as the data type of admin_metadata has been changed from string to json. I would assume this will be part of the UI update from @CannonLock
• Note: This PR has foreseeable merge conflict with Require Admin approval for new caches #439 / Namespace registry admin #462 . Depending on which PR gets merged first, the other PR requires non-trivial conflict resolution. @turetske We can talk about how to fix the conflict.

[bbockelm]

A modest number of TODO items for the review -- the most important to focus on is the institutional validation.

[bbockelm]

LGTM!

I'm sure there are additional items I've missed but I like the conceptual design, we already went through the first round of issues, and I worry about merge conflicts across such a large change.