Consistency in js and css file locations

(This is for issue #205).

* I moved the folder containing all of the Sphinx Bootstrap theme related files from `static/plugin_layouts/*` to `source/_templates/plugin_layouts/*`. I think it makes more sense to have everything related to the pages generated by Sphinx in the `source/` folder, and keep everything related to the web2py generated views in `static/`. (This doesn't really affect the ability to zip up the static assets for a course and serve them from anywhere.)

  This means to modify the layout of the Sphinx-generated pages, you'd change `source/_templates/plugin_layouts/sphinx_bootstrap/layout.html`. To modify the layout of web2py-generated pages, you'd still edit `views/layout.html`.

* Used a symlink to handle needing our `runestone-custom...css` file in both Sphinx- and web2py- generated pages. The master/original copy still resides in `static/`, and there is a link pointing to it in `source/_static/`. My experimentation so far shows that git handles it without a problem. You can open and modify it from either location, and git correctly versions the master copy. (The one issue that may pop up is that symlinks aren't supported on a FAT32 filesystem. I doubt many developers are still on FAT32 however.)

* Moved the 2 JS files so they are in the correct location.

* Changed how the RSI logo is being added so that the stylesheet can be served from anywhere.

* Removed the files related to the old Abrasive theme.