Search for migrated @ mentions does not work, migrated CC ineffective

[mkoeppe]

As reported by @mezzarobba in https://groups.google.com/g/sage-devel/c/jrYmRl4WSxM/m/3pZ07NHOAgAJ:

Another one (more of a warning to other users actually): apparently
subscriptions to issues have not been migrated; you need to
re-subscribe to issues you are interested in.

Maybe related: search patterns of the form mentions:someone (including
the default filter 'Everything mentioning you') seem to only return
issues created after the migration. (author:someone works fine.)

...However, involves:@me seems to work.