Add Excerpt field to be used with Jetpack's Publicize

When used with Jetpack's Publicize module, users are unable to set an excerpt and instead, job description's first 55 words (this varies) are used when shared to social media.

We can add the Excerpt field and enhance the WP Job Manager + Jetpack combo.

Came up in JP 2866813-t

The issue names no files or tests; start by tracing how WP Job Manager defines and saves job listing fields, then inspect the Jetpack Publicize sharing path. Done means users can set an Excerpt independently and Publicize uses it instead of the generated first 55 words.

Written by the indexing model from the issue text.