Task: Create GitHub milestones based on GSoC deadlines

[pllim]

I think it would be beneficial to:

1. Create milestones from relevant GSoC 2019 deadlines: https://developers.google.com/open-source/gsoc/timeline
2. Assign milestones to all the issues and pull requests.

This way, even without a project board, we can guesstimate the work load and whether we are ahead or behind in the project.

[tcjansen]

Sure - I might need help estimating which deadlines should be assigned to which enhancements/tasks.