project timeline view

Show a timeline view of everything happening in the project. Item status
changes, comments from users, hours logged, forum posts,
status updates, and milestones hitting target dates.

All merged into one reverse chronological view.

Ie, sort of a facebook timeline for a project.

I've kept the timeline stuff fairly abstract so that most of it can be reused
so that we can do user timelines and probably a global timeline. Starting
with just per-project for now though.

I've put the link to the timeline page behind the 'project_timeline' waffle flag
which should currently only be enabled for admins. Once we've played around a bit
and probably gotten some design input, we can open it up.