Add support for milestones in task editor

[devsprint]

Task editor should allow an user to view the list of existing milestones, change the milestone associated with a task and to be able to add new milestones.

A milestone may have 0 to n task associated.

[devsprint]

Depending on Github Issues 2.0 API availability.

[devsprint]

https://gist.github.com/71c2878f53e886dc921a - new api