Add Job Descriptions to Email Templates

**[Original report](https://bitbucket.org/laborstudents/lsf-flask/issue/293) by Alex Bryant (Bitbucket: [Alex Bryant](https://bitbucket.org/Alex Bryant), ).**

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We want to add the job description of the position to be added to the email template that goes to students and supervisors when a student is hired. We also want to save job descriptions historically \(i.e., the Spring 2020 job description for position X can be different than the Spring 2021 job description for the same position X\)