[re #12346] Added support for saving severities via custom fields in Jira

• Added support for retrieving Severity via Jira custom fields - as discussed with Mr. Pícha
• Right now, if the custom field with name "severity" is not linked to the issue, the default value null is set
• Otherwise, we extract id and value of that severity and assign it to work_unit item
• Removed also unused methods from jiraRestClientService

Note: after demo review with Mr. Pícha, I will delete the unsused code just to make sure we used the "right" implementation