Bonsai task management: GitHub Issues

The various suggested Bonsai tasks are currently described primarily in the Wiki (plus [here](https://chris.mutel.org/bonsai-components.html) and [here](https://github.com/BONSAMURAIS/hackathon-2019/blob/master/Agenda.md)). The GitHub wiki interface is not designed for task management: it lacks discussion, deadlines, dependencies, workflows, task completion, archiving etc. So there is room for improvement here.
GitHub Issues [are designed](https://guides.github.com/features/issues/) for this. @romainsacchi and I have discussed and researched this topic. We also considered GitHub [project boards](https://help.github.com/en/articles/about-project-boards), but they mostly build upon and extend issues. We think that the tasks listed in the Wiki should be converted into Issues. This should make the Bonsai requirements, status, management, and development speed more visible, integrated and consistent, and help with decision making, provenance, and getting new users started with contributions.

Requirements:
- [x] copy some example tasks from [here](https://github.com/BONSAMURAIS/bonsai/wiki/Data-Storage)
- [x] create [Bonsai-specific Labels](https://github.com/BONSAMURAIS/bonsai/labels) for Issues
- [x] communicate this suggestion to the mailing list

If general agreement on this approach is reached:
- [ ] copy across the remaining tasks from other locations
- [ ] delete the duplicated tasks from the Wiki
- [ ] update the [getting started Wiki page](https://github.com/BONSAMURAIS/bonsai/wiki/Getting-started) to refer to high-priority tasks

Aside: I considered starting a [BEP](https://github.com/BONSAMURAIS/enhancements/blob/master/beps/0002-bonsai-project-community-governance-structure.md) for this but thought the administrational overhead was too high. So I instead opted to make the changes as a proof-of-concept, then gather feedback after.