Transition To Pagestore Model

[JD-P]

Instead of holding posts in a database, it would be desirable to use a flat-file pagestore model so that posts can be managed by git or other version control systems. In order to do this, several things have to happen:

• Every reference to posts and comments in the source code has to be converted to use the pagestore API
• Some idea of how to reference pagestore items from the API needs to be in place (i.e, what ID system to use for pages)
• It should be possible to put the page store API in top of arbitrary backends using a python wrapper, which implies that page stores need to be swappable and loadable as extensions.

And probably other stuff that I'm forgetting right now.