fix($core): cannot retrieve the correct theme name when them path is a local absolute path linked to a javascript file.

We shouldn't let the core render the `.content` class, so once we use custom layout in the default theme, the styles of default theme will pollute the styles of custom layout.

This reverts commit 760f90b9.

This reverts commit cab5b831.

fix($core): failed to load theme when using "layouts/Layout.vue" as entry file (close: #1563) (#1564)

fix($core): use directory name to compute slug if filename is readme or index (close: #1443) (#1535)

BTW, A theme cannot require the theme's styles at the layout but need use `Index.styl`  instead.

Also renaming __VUEPRESS_ROUTE_BASE__ to __VUEPRESS_ROUTER_BASE__

Setting it via window.__VUEPRESS_ROUTE_BASE__ at enhance app file.

ref: https://github.com/CompuIves/codesandbox-client/pull/1652