Add navigation

[joshsmith]

This adds the beginning of navigation. No tests, so there's lots to be done here. Could use plenty of help.

[joshsmith]

[begedin]

To do

• Integration tests for user-menu under integration/components/user-menu.js
• Acceptance tests for home page
  • When logged out
  • When logged in
    • User menu visible
    • User menu links work

We need acceptance tests because integration tests seem to not have access to the router, so the links don't have the proper URLs.

Fix issues with reliance on typeKey instead of modelType/ownerType

Mirage does not create actual model instances. Instead, it creates instances of its own model class. This means there is no typeKey, which is why I was using a manually set modelType property, which would now, I guess, become an ownerType property. The API already has this property, due to the nature of the polymorphic relationship, so it was just a matter of adding it to the serializer. This is what Mirage was relying on up to this point.

Additionally, due to the same reason, Mirage can't handle computed properties defined on the model. They don't get transferred over to the mirage model. For that reason, I used computed properties on the component level.

In order to resolve this we need to

• Replace all referrences to modelType with ownerType
• Revert back to using computed properties on the component level instead of model level

I'm afraid it's not a nice solution, but it's the only one I can think off.