Combine separate description, short description fields on sessions

Problem/Motivation

Event Platform currently has separate description and short description fields on sessions. In practice, however, it appears that the Event Horizon theme is designed to show the full description, even in teasers. If the short description isn't used anywhere, we should revisit if there is value in keeping this as a separate field.

Proposed resolution

Combine both into an upgraded version of the standard Body field. It is possible to make the summary required for sites that do still want a short description, and solutions like Summary Options → and Smart Trim → can provide extra flexibility for the setup of a Body field on sessions for sites that want to use a shorter format.