MAke github.py works correctly with PATCH and PUT methods
Worker to fetch non-existing repositories when subscription added
Don't fail when fetching repo with issues deactivated
Add an "empty" message if no labels in the repo dashaboard
Add a github_status field to keep create/update/delete state
Unused import on tasks/githubuser.py
Save the api url of labels to be able to edit them

(Because the url is based on the name which may have been changed by the
user)
Add a dist_edit method to create/update github objects
Add a job to create/update labels when done in the labels editor
Add a dist_delete method to delete an object on the github side
Update (and use) the LabelEditJob to manage deletion of labels