[re #12345] Migrated attachment, user & category services, fixed big in core service method

In this MR:

• the bug in CategoryService in core package was fixed
• Migrated another services - User, Attachment and Category (and also optimised - they are being executed at each issue during the processing issue in JiraWorkItemService - thanks to that, we again reduce the Jira API calls)
• Added missing comments to " fully finished" methods
• Changed the way how to pass the url - now the URL can be retrieved from JiraUrlUtils - that enabled me to build URLs to each issue and save them into work_item table