[site] If base_url ends with /, the theme links don't work

[preaction]

When I added a base_url that ended in /, the stylesheet links ended up like:

<link rel="stylesheet" href="//theme/css/normalize.css" />
<link rel="stylesheet" href="//theme/css/skeleton.css" />
<link rel="stylesheet" href="//theme/css/statocles-default.css" />

Which clearly does not work.