Support for multi-session identities

Currently, MediaGrid only supports ephemeral identities. After 24 hours, or clearing the browser's cookies, an identity is gone and cannot be re-used.

Allow for users to set their own passwords and log into an existing identity.

It would be good to have a setup script that will prompt the administrator if they want support for long-term identities or not.

Start by locating the existing ephemeral identity creation and login flow, then find the setup script or administrator configuration entry point. Define how passwords and identity persistence should work across browser-cookie loss and expiration, and confirm that administrators can enable or disable long-term identities before considering the feature done.

Written by the indexing model from the issue text.