Factor out library builtin pages for reuse

[pfrazee]

The library pages have utility for the dat CLI, web extensions, and (probably) many other places. It'd be great to factor them out so that they could be reused more easily.

Probably would require:

• Refactor/reorg code into its own repo. (A good time to do cleanup.)
• Come up with an API for different environments to leverage the pages.
• Build tooling.
• Review any custom APIs used and see if they can be improved.
• Create a process for injecting the custom API implementations which fit the environment using the builtin pages.