Remove double slashes from media_folder/public_folder

[erquhart]

Is your feature request related to a problem? Please describe.

There's bound to be confusion and mistakes around our cascading media_folder/public_folder string replacements. If your global value ends with a slash, and then you add a slash after including that value as a string replacement, you get an unexpected double slash:

media_folder: /static/img/

collections:
  - name: blog
    media_folder: "{{media_folder}}/blog"

Describe the solution you'd like

This potential mistake is simple to anticipate and account for, so we should. When constructing the final value of either property, double slashes (//) should be replaced with a single slash. Because media_folder/public_folder are not used for complete urls, they won't be affected.