Only add the service user to the index once (lazily)

Adding and removing it again with each ListAccounts() call was a huge
overhead. This is a temporary workaround, the whole service is gonna be
replaced by the idm service soon anyway.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f