Make MIME-type icons monochromatic

As part of CMS-8565, Anton created new SVG icons for different mime types in the CMS' assets section. These where added to the Hippo theme as part of HIPPOTHEME-104.

Some of these icons contains text (such as "zip", "odt" etc.) which is "hardcoded to the color white (a second color on top of the "fill" color). When these icons are displayed for a selected item (blue background, white foreground), these text labels disappear, because they are white, too.

The SVGs should be adjusted such that the text is not using a hard-coded color (fill="#fff" inside the SVG file) but rely on the value of the fill property provided by CSS.

One of the drawbacks of this approach is that the text will only be visible if the SVG is rendered with an explicit CSS rule defining the 'fill' color. See below for an example, notice the 'hi-fill-white' class on the SVG element.

  <svg name="mimetype-docx" size="xl" class="hi-fill-white hi hi-mimetype-docx hi-xl"><use xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#hi-mimetype-docx-xl"></use></svg>