Add description and confidential fields to Epic new work item

Add some stuff to new New epic page.

• show Toast after success instead of redirecting
• Validation on title field instead of disabling Create button
• Rename some things (workItemTypeName and Id instead of workItemType

How to test

1. Enable namespace_level_work_items feature flag
2. Go to Epic list page for a group
3. New Epic
4. Fill out the form