Add JImmAdmin field to User struct

[kian99]

Description

Slight refactor to make our code more DRY. Instead of repeating checks like the following,

isControllerAdmin, err := openfga.IsAdministrator(ctx, user, j.ResourceTag())
	if err != nil {
		return errors.E(op, err)
	}

in every facade handler, we instead do the check once when we authenticate the user and pass this information along with the user.

Also made various function signatures more consistent by calling the user variable just u. In some places it was user and others u.

The main issue with this change is that it heavily affects our tests. Tests inside jujuapi are largely unaffected because these tests act like integration tests, opening up a client connection as any external client would. Tests inside the jimm package however are more deeply affected as they call JIMM's handler methods directly and expect the methods to perform an OpenFGA check to determine if the user is a superuser, so these tests required some tweaking to pass the user's admin info from the test itself. Overall I think this is actually a good thing as we can move closer to making jimm tests purely unit tests (though some methods still make calls to OpenFGA which we should consider mocking out), and then place integration tests inside jujuapi.

Engineering checklist

Check only items that apply

• Documentation updated
• Covered by unit tests
• Covered by integration tests

[alesstimec]

LGTM.. but.. i don't thing renaming user to u was worth it.. it just makes the PR harder to review and brings little to no value - i think calling variable user is better than u..

[alesstimec]

why? saving 4 characters is hardly worth it :)

[kian99]

Well the nice thing about having it consistent is that it's easier to rename now. So I'll change it to user.

[ale8k]

I agree renaming u to user makes sense honestly

[ale8k]

Makes sense to me, good job

[ale8k]

lgtm, nice work

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