Add custom served folders

[WithoutPants]

Resolves #455

Allows configuration of mapped URLs to file system folders. These are served from the /custom url prefix. For example, in config.yml:

custom_served_folders:
  /: D:\stash\static
  /foo: D:\bar

With the above configuration, a request for /custom/foo/bar.png would return D:\bar\bar.png. The / entry matches anything that is not otherwise mapped by the other entries. For example, /custom/baz/xyz.png would return D:\stash\static\baz\xyz.png.

This configuration is not exposed in the UI to minimise possible malicious use.

Applications for this include using static images in custom css, like the Plex theme. For example, I used the following config:

custom_served_folders:
  /: <stash folder>\custom

I placed background.png and noise.png in the custom folder, then in the custom css I put:

body {
	background-image: url("./custom/background.png");
...
}

#root {
	background: rgba(0, 0, 0, 0) url("./custom/noise.png") repeat scroll 0% 0%;
...
}

Other applications might be to drop UI extensions into this folder, allowing people to extend the UI further. I tried putting the ui/v2.5/build directory into my configuration, but it's built to use specific URL paths, so didn't quite work correctly.

[bnkai]

Tried using the css and accessing the folders directly from browser. Works ok.
Another trick you can do is link the error.log ( i used soft links) to the custom folder and so have access to your whole log from inside stash.