Refactor validations in JiraController

What does this MR do?

It refactors validations in JiraController. Now we use JiraImport.validate_project_settings! method instead of individual validations called from the controller.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.