Base template changes

- Shifted the project's main stylesheet out into the `main_css` block, which allows it to more easily override lib css which tends to be dumped into `css`.
- Cleaned up some mixed indentation in the comment block describing the different blocks available from ixc_core/base.html.
- Changed the low-level elements to use classes rather than ids, and changed the selectors to be both more verbose and to use camel-case.