`page.collection` contains the collection slug which is `posts` for,
well, the built-in posts collection.
`collections.posts.pages` is a sorted list of all of the pages that are
part of the posts collection.
Fixes#323
BREAKING CHANGES:
`page.is_post` is replaced by `page.collection`. Generally, it'll look like
`{% if page.is_post %}`
and can be replaced by
`{% if page.collection == "posts" %}`
`posts` is replaced by `collections.posts.pages`. Generally, it'll look
like
`{% for post in posts %}`
and can be replaced by
`{% for post in collections.posts.pages %}`
`cobalt migrate` should do this for you.
