ringaccount: save registered name in configuration

Use the account configuration to store the registered name,
allowing to know the account registered name for disabled accounts,
before account registration happens,
or if the name server is not reachable and the name cache
was cleared.

This allows to show the account registered name instead of
the RingID more consistently in clients. The field is not
authoritative as information from the name server always
take precedence.

Change-Id: Ief8725da57d609ce9bbaeda6b2d564638ea3b191