SqlExpressions: Panels break when using SQL Expression with frontend data source

When using a frontend data source, or Mixed data source that includes a frontend data source, panels break due to lack of SQL Expressions support for frontend datasources. This issue captures the overarching problem that will be addressed in various subtasks. [Design Doc: SQL Expression applicability](https://docs.google.com/document/d/1EIUpmuh29PivxP2dxHSeG7Nturb5rSMKWAehJLV2JeY/edit?usp=sharing) captures the current behavior, a short-term frontend fix, and a more holistic backend fix.