Implement themes/skins

This MR allows to have different themes (or skins) for Kresus. Here is what it does:

• Specify the wells colors in the CSS only (no more colors defined in JS) as CSS variables (which will also allow to use the color on the well or on one of its children if needed)
• Move all the current fonts/images assets in static/ to client/themes/default
• Ship the theme and the base CSS (bootstrap + components + kresus structure & sprites) separately and output all themes in the build/client on build
• Allow to define the theme used in the settings in database
• Allow to choose the theme to use from the UI
• Add a spinner in the index.html file (while the /app request loads)

The last commit includes my own WIP of a lighter skin. I can commit it but I'd prefer to host in on my own repo for easier updates.