Add scaffold for Controls

[csutter]

This adds a basic CRUD scaffold for controls.

Controls are rulesets defined in Search Admin (and eventually synchronised to Discovery Engine/Vertex AI Search in a future change) that allow us to override the results returned.

In theory, these controls can be quite complex, but our MVP will probably approximate the previous "best bets" functionality where search administrators can promote certain documents to rank higher for a given query ("boosting").

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[leenagupte]

So the basic scaffolding code looks good. 🎉

What I'd really like to understand how Controls are going to work going forward. I think I'm missing some context as I'm not sure why this is being added now without the something that uses it, or if the "something" is being workedhe on right now.

So everything, (Best bets, etc) apart from Recommended (external) links will be stored as a Control? Will recommended links eventually be moved to the Control model, or will they stay separate?
Does that mean admins will be able to select from a list of Control types to add or view something?

At the moment, the Search Admin home page redirects to the "External links page". Will change too?

P.S. The changes was really easy to follow and review ⭐ and thanks for adding screenshots 🥇

[csutter]

Closed in favour of #1254 following a new design doc, which also incorporates some minor improvements.