Change default autofollow_on_join_user

Due to performance of joindiaspora.com, a decision was made to set up a project pod that hosts the official support and news autofollow account (see https://www.loomio.org/d/tyZdcGNo/diasporahq-account-move-to-a-project-pod). The new account is hq@pod.diaspora.software.

closes #6369