Ensure there's a trailing newline on each user identity info entry.

Follow-up to !2318 (merged)

Without this change the file periodically ends up with two entries on the same line, making the file much harder to parse.

Example of the broken file:

{"gitterUserId":"abcdef","mxid":"@alice-abcdef:gitter.im","provider":"github","providerKey":"1234"}
{"gitterUserId":"ghijkl","mxid":"@bob-ghijkl:gitter.im","provider":"github","providerKey":"5678"}{"gitterUserId":"mnopqr","mxid":"@charlie-mnopqr:gitter.im","provider":"github","providerKey":"9123"}

This change should fix the broken lines (although I have no way to test it), I believe this now conforms to ndjson.

Related to #2856 (closed).