Allow for visibility on speaker attributes during moderation, scheduling

Problem/Motivation

Some camps want to make sure they are prioritizing sessions by first-time speakers or speakers from marginalized groups.

Proposed resolution

Adding filters based on user (speaker) fields would be easy enough, provided the fields exist. That may be more of a documentation issue until we decide to create the fields on the users as part of the default install.

On the scheduler, we could also allow for user fields to be shown as filter options, and then update the code that generates the sessions and the filters accordingly.