minify: rebase relative urls in imported files.

4177b3f moved the font-face declarations from src/static/css/pad.css to two
imported files (src/static/css/pad/fonts.css, src/static/css/pad/toolbar.css)
in a different directory.

This results in the font files being invoked from CSSes residing in different
directories in the minified and un-minified case. URLs in the src attribute are
relative to the stylesheet path [0], and so we have to start requiring clean-css
to rebase them.

Before this change, the non minified casse worked by chance, because there were
a lot of "..", which ended up resolving to the root of the site anyways.

Fixes #3956

[0] https://developer.mozilla.org/en-US/docs/Web/CSS/@font-face/src

src/node/utils/Minify.js

@@ -426,17 +426,17 @@ function compressCSS(filename, content, callback)
     /*
      * Changes done to migrate CleanCSS 3.x -> 4.x:
      *
-     * 1. Disabling rebase is necessary because otherwise the URLs for the web
-     *    fonts become wrong.
+     * 1. Rework the rebase logic, because the API was simplified (but we have
+     *    less control now). See:
+     *    https://github.com/jakubpawlowicz/clean-css/blob/08f3a74925524d30bbe7ac450979de0a8a9e54b2/README.md#important-40-breaking-changes
      *
-     *    EXAMPLE 1:
-     *        /static/css/src/static/font/fontawesome-etherpad.woff
-     *      instead of
-     *        /static/font/fontawesome-etherpad.woff
-     *    EXAMPLE 2 (this is more surprising):
-     *        /p/src/static/font/opendyslexic.otf
-     *      instead of
-     *        /static/font/opendyslexic.otf
+     *    EXAMPLE:
+     *        The URLs contained in a CSS file (including all the stylesheets
+     *        imported by it) residing on disk at:
+     *            /home/muxator/etherpad/src/static/css/pad.css
+     *
+     *        Will be rewritten rebasing them to:
+     *            /home/muxator/etherpad/src/static/css
      *
      * 2. CleanCSS.minify() can either receive a string containing the CSS, or
      *    an array of strings. In that case each array element is interpreted as
@@ -447,7 +447,13 @@ function compressCSS(filename, content, callback)
      *    "content" argument, but we have to wrap the absolute path to the CSS
      *    in an array and ask the library to read it by itself.
      */
-    new CleanCSS({rebase: false}).minify([absPath], function (errors, minified) {
+
+    const basePath = path.dirname(absPath);
+
+    new CleanCSS({
+      rebase: true,
+      rebaseTo: basePath,
+    }).minify([absPath], function (errors, minified) {
       if (errors) {
         // on error, just yield the un-minified original, but write a log message
         console.error(`CleanCSS.minify() returned an error on ${filename} (${absPath}): ${errors}`);