Use REST endpoint for subtask creation dialog

[codescape]

We can get rid of the workaround for Jira 10 and make use of the REST endpoint to create Subtasks from the subtask creation dialog.

This possibly also enables us to improve the interface as mentioned in:

• Increase the size (width) of the dialog popup to have more space #52
• Allow to move the dialog over the screen #119

Tasks

• create empty dialog with REST support
• consume REST endpoint to create subtasks
• add license check to REST endpoint