Add basic job details to email notifications

[jom]

Fixes #672
Builds off of #1422

Changes proposed in this Pull Request:

• Adds basic job details to email notifications. I chose just a few of the fields to put in for now, but we can add more later if desired.

• Refactored a bit of what I just did in Add email styles and header and footer #1436 to ease future complexity with filters and hooks (cdde24f).

• Uses new stylesheet functionality from Add email styles and header and footer #1436.

• Cleans up new job listing email notification.

• Adds wpjm_get_the_job_categories() and wpjm_the_job_categories () in support of this objective.

Testing instructions:

• Ensure all tests pass.
• Submit a job listing using the [submit_job_form] shortcode and verify the admin gets an email with the correct job details.

Proposed changelog entry for your changes:

[alexsanford]

This looks really good! I tested it out, and it works well. 👍