Setting to automatically upgrade old signed base64/JSON URLs

[simonw]

Have an optional upgrade path (controlled by a setting, off by default) where if a signed ?sql= string appears to be a base64 encoded JSON object with a valid signature we decode that and redirect to the new-style raw SQL URLs

Originally posted by @simonw in #45 (comment)

[simonw]

It's DASHBOARD_UPGRADE_OLD_BASE64_LINKS = True in settings.py.