Add support for Sprint and Epic Link fields

[aldenquimby]

Agile JIRA Software projects have Sprint and Epic Link picklist fields:

But in the sentry Create JIRA Issue form, these appear as plain text fields:

In JIRA's custom field configuration, these are LOCKED (not editable), because they are part of the agile software project workflow:

It would be great if these showed up as picklists in the sentry form.

[davidemerritt]

+1, the epic_link field requires you to reference the custom_field id which no one knows.