Add missions.

There are Missions and then there are Plots. Each Mission should have
one Plot for each team.

Test coverage is still pretty bad, and I have done absolutely no
styling.
Replace outcome node with in-template logic.

Something something lesser of two evils.
Authentication

So humans and zombies can't see each others' mission stories.
Add a visibility flag.

Also, actually enforce mission visibility this time.
Merge pull request #129 from jthemphill/feature/missions

Add missions