Bug 1030972 - Import tasks from Bugzilla

@bobsilverberg @Osmose r?
You have added `is_valid` in this PR, but have not updated the app to address it. For example, tasks listed as invalid still appear in the Available Tasks list. There are a number of places where we need to deal with this new `is_valid` attribute which have not been addressed. As this is not critical to this PR, I suggest pulling these changes out and creating a separate PR which implements `is_valid` and all of the code necessary to make it work.