Colorize roadmap using Bootstrap alerts

This MR introduces new colors to Bootstrap for Cloud, Software, Backbone and Overall. These colors are then used to colorize the roadmap.

Closes #8 (closed)

_sass/_bootstrap_variables.scss

@@ -11,10 +11,10 @@ $theme-colors: (
     "warning":          $color-helmholtz-energy,
     "overall":          $color-helmoltz-gray,
     "backbone":         $color-helmholtz-matter,
-    "cloud":            $color-helmholtz-space,
+    "cloud":            $color-helmholtz-earth,
     "software":         $color-helmholtz-green,
     "overall-light":    lighten($color-helmoltz-gray, 30%),
     "backbone-light":   lighten($color-helmholtz-matter, 15%),
-    "cloud-light":      lighten($color-helmholtz-space, 15%),
+    "cloud-light":      lighten($color-helmholtz-earth, 15%),
     "software-light":   lighten($color-helmholtz-green, 15%)
 );