[Feature] Support configurable static folder

[WithoutPants]

Is your feature request related to a problem? Please describe.
Allowing the user to configure a "static" folder would allow css themes to reference files outside of the stash binary and will also allow UI extensions or replacements. A simple example would be to add a background image to the custom css with the URL from the static folder. A more complex example might be if someone coded a replacement UI and placed the files within the static folder.

Describe the solution you'd like
Add a (non-exposed) configurable static folder (or folders). This folder will be accessible from the front-end under the URL /static. May want to consider whether this folder should be accessible with or without logging in or if it should be an option.