Fix: handle expiration of refresh token

[ottokruse]

Issue #, if available: N/A

Description of changes: When the ID and Access tokens have expired, and the Refresh token too, the user is now redirected to the Cognito Hosted UI to sign-in. Previously, this resulted in the custom Auth@Edge error page to be shown, which made the user explicitly click the sign-in button to be redirected to the Cognito Hosted UI to sign-in. That page should be shown for edge cases only.

Implemented as follows:

1. RefreshAuth function: if the refresh fails with invalid_grant message from Cognito, we assume the refresh token expired and clear the cookie
2. CheckAuth function: if the ID and Access token are expired and there is no refresh token (because it was now cleared), the user is redirected to Cognito for sign-in

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[shanant321]

LGTM