celery: fix send_email to work with JSON encoding (Issue #363)

Long time ago, c935bcaf7086 introduced an optional User object parameter to the
send_email task and used the computed full_name_or_username property. Due to the
magic of pickle, that also worked when using Celery to run the task async.

Now, Celery 4 changed the default encoding from Pickle to JSON, which we
anticipated in e539db6cc0da. That broke send_email in some cases, for example
when a user comments on another user's changeset.

Fixed by passing the "From" name as string instead of passing the whole User
object.

Thanks to vyom for reporting.