Allow re-using a UI auth validation for a period of time (#8970)

changelog.d/8970.feature

docs/sample_config.yaml

synapse/config/_base.pyi

synapse/config/auth.py

synapse/config/homeserver.py

synapse/handlers/auth.py

synapse/rest/client/v2_alpha/account.py

synapse/storage/databases/main/registration.py

...es/main/schema/delta/58/26access_token_last_validated.sql

tests/rest/client/v2_alpha/test_auth.py