Convert @ in usernames to @ for improved identitiy matching

[aklapper]

The exported JSON file of mediawiki-identities, many identities from the source "wikimedia:its" (=Bugzilla) have a literal @ in their username (Bugzilla usernames are email addresses).

Other "identities" have the very same email addresses but with a literal @.

The software should convert @ into @ for better identity matching across different sources.