Add "Sprint Planning" View

[adewes]

Currently, the app is mostly for viewing issues, while interaction takes place mostly on Github. While this is a reasonable approach, for some use cases it would be great to be able to manipulate issues directly through Gitboard.

One such use case it the planning of sprints: For this, we usually take existing issues (that are either in the backlog or associated to another milestone) and want to add them to a given milestone / sprint.

We should implement a view where the user can create and populate a new milestone with existing issues from Github.