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

@@ -3,10 +3,18 @@
 $border-radius:    0.25rem;
 
 $theme-colors: (
-    "danger":       $color-helmholtz-health,
-    "info":         $color-helmholtz-space,
-    "primary":      $color-helmholtz-blue,
-    "secondary":    $color-helmoltz-gray,
-    "success":      $color-helmholtz-green,
-    "warning":      $color-helmholtz-energy
+    "danger":           $color-helmholtz-health,
+    "info":             $color-helmholtz-space,
+    "primary":          $color-helmholtz-blue,
+    "secondary":        $color-helmoltz-gray,
+    "success":          $color-helmholtz-green,
+    "warning":          $color-helmholtz-energy,
+    "overall":          $color-helmoltz-gray,
+    "backbone":         $color-helmholtz-matter,
+    "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-earth, 15%),
+    "software-light":   lighten($color-helmholtz-green, 15%)
 );