Rename "pages" to "singles"

[kaidesu]

To avoid confusion between a matrix type of "page" and creating a collection for "pages", it makes sense to rename these to "singles". This also brings the terminology inline with the term "collections".

So with Matrices, we'll have two types:

• Singles, for top-level and one-off content
• Collections, for a group of related content (e.g. blog posts)