Re-authentication before performing security-sensitive ops

[nvoutsin]

Following #384, an optional step-up authentication mechanism should be introduced to enhance session protection and add additional safety measures before accessing higher-security areas of the app. Re-authentication may be triggered in various situations, such as: (a) before issuance, when reusing an access token; (b) before sharing data with verifiers; or (c) before other critical operations in the settings menu, e.g., account deletion (see also #317)