Remove `active` from Url model

This is basically a revert of commit b799bc2af. We don't need this
configuration anymore (plus, it wasn't being used).

Conflicts:
	app/models/url.rb
	db/schema.rb
Revert "Add cron job to create snapshots"

This reverts commit e8f5fbb80. We don't plan to use cron jobs to trigger
snapshots.

Conflicts:
	Gemfile
	lib/tasks/diffux.rake
Simplify `SnapshotComparer#compare!` and add specs

I found a couple of cheap simplifications to the code and added specs to
cover some basic behavior.