Make LoggedOut page wait to redirect until iframes are loaded

[dherr]

Addresses issue #3212, where PostSignOutAutoRedirectDelay=0 often causes the page to redirect before the single sign-out iframes have loaded. Increasing PostSignOutAutoRedirectDelay prevents the error, but involves a trade-off between allowing for slow connections and causing unnecessarily long delays.

This PR detects when the iframes are loaded by adding onload event handlers using an angular directive. When PostSignOutAutoRedirectDelay==0, it redirects when all iframes have loaded. When PostSignOutAutoRedirectDelay > 0, it will force a redirect after the delay has passed, but will redirect sooner if the iframes are finished. This maintains existing functionality by allowing a maximum delay to be specified, while otherwise providing the fastest possible sign-out flow.

A similar approach is used in IdentityServer4 by redirecting on window-load. Since we need to use IdentityServer3 for a while longer, I was looking for a way to accomplish the same thing using its angular-generated signout iframes.

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[dnfclas]

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