Use service user for role assignment and autoprovisioning

This gets us a rid of the need to configure the reva jwt secret in the
proxy. Also we no longer need to fake an internal admin user for
autoprovsioning user and/or assigning the roles to users from oidc
claims.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f