Basic gsoc timeline, with milestones events.

Needs ability for project members to publish periods of extended unavailability, if they want to, before and during gsoc.

Also can be used to record significant project related events / posts, such as software releases, survey result announcements, cEP merges which affect everyone (e.g. gsoc responsibilities changes, or CoC updates).

gci timeline would also be nice. It could only roughly cover events that happened in GCI 2017 retrospectively, as GCI 2018 is a long way off. Maybe this should be left for a follow up issue.

Ideally we use standard jekyll features , and plugins.