Add the CSS for the top menu enhancements (common to both main wiki and workspaces)

Since both the main wiki and the workspaces have to have new elements, we need a CSS that's visible by both. A StyleSheetExtension can be made visible to an entire wiki at most, but not globally.

There would be 2 options:
1. To avoid redundancy, extend style.css with an extra @import to a page in the main wiki that has a StyleSheetExtension and that is actually used only when it's present.
2. Use a page that has a StyleSheetExtension set to wiki-level visibility and duplicate it into the main wiki and all of the generated workspaces. This is less invasive, but harder to maintain and redundant.

Currently, I`ve chosen option 1 by doing something like:

#set ($workspaceManagerStyleDocumentReference = $services.model.createDocumentReference($xcontext.mainWikiName, 'WorkspaceManager', 'Style'))
@import "$xwiki.getURL($workspaceManagerStyleDocumentReference, 'ssx', '')";