@user mentions should not be case sensitive

Summary

When users are mentioned in the various fields, capitalization of the username is required in order to provide correct linking.

Steps to reproduce

Enter @SeanPackham vs @SeanPackham into a field and watch as one gets linked and one doesn't.

Expected behavior

Any @username should be displayed irrespective of case, given that we do not honor case as a means of differentiation between account names (i.e. seanpackham is unavailable because SeanPackham is in use)

Possible fixes

All uses of username expansion and linking should have .downcase called on them.