Validate post contents when editing

chapter9-5
Added comments collection, pub/sub and fixtures.

chapter10-1
Display comments on `postPage`.

chapter10-2
Created a form to submit comments.

chapter10-3
Made a simple publication/subscription for comments.

chapter10-4
Denormalized the number of comments into the post.

chapter10-5
Added basic notifications collection.

And create a notification when a comment is created.

chapter11-1
Display notifications in the header.

chapter11-2
Only sync notifications that are relevant to the user.

chapter11-3
Added enough posts that pagination is necessary.

chapter12-1
Augmented the postsList route to take a limit

chapter12-2
Refactored postsLists route into a RouteController

chapter12-3
Added nextPath() to the controller and use it to step through posts.

chapter12-4
Add a spinner to make pagination nicer

chapter12-5
Use a single post subscription to ensure that we can always see the right post.

chapter12-6
Added basic upvoting algorithm.

chapter13-1
Grey out upvote link when not logged in / already voted.

chapter13-2
Added pluralize helper to format text better.

chapter13-3
Better upvoting algorithm.

chapter13-4
Added routes for post lists, and pages to display them.

chapter13-5