Create django command to extract non-issue tasks as a separate yaml file

http://community.coala.io/static/tasks.yaml contains all tasks from 2017 which were published.

They can be pulled into the django system using django command `fetch_gci_task_data` . .ci/build.sh puts them into `_site`.

Create a django command which loads all the tasks, and filters out any of the tasks which have an `external_url` which is an issue, and creates a new yaml file of the filtered list of tasks.