Split `app/templates` into its own tree.

Currently, we are processing the entire `app/templates` and `app/styles`
directories for all `preprocessJs` filters (this means things like
`ES3SafeFilter` are actually checking each template).

This makes `templates` it's own tree, and splits things up
appropriately.
Merge pull request #1238 from rjackson/split-templates-tree

Split `app/templates` into its own tree.