BE: Retrieve details for individual Jira issues

As a developer using the Jira issue list in GitLab, I want to be able to view the details of a specific issue in the list, so that I can get the full context of the work I'm doing.

This should be implemented using an individual's user credentials for Jira.

This MVP iteration is just adding the ability to fetch data from Jira for a single issue and translate the result into a format that can be used by the frontend to display the issue.