Bootstrap4 themes (#749)

I updated bootstrap-theme, bootswatch and material design themes to bootstrap 4.
The biggest change is that oficial bootstrap 4 does not provide `bootstrap-theme` as separate stylesheet rather we have to enable two properties in sass source and compile it. I added shell script that automates this process, similarly to scripts in other themes.

Remaining themes are in different state of migration:

* M8tro - No sign of upgrade to 4 (idleberg/m8tro-bootstrap#4)
* Vegibit - No sign of upgrade to 4
* Google - Upgrade in progress (todc/todc-bootstrap#172)

What do you think is the best approach here? Can we just delete them? Or should we wait until todc releases new version of Google theme before we merge this to upstream branch?

bootstrap-themes/src/main/java/de/agilecoders/wicket/themes/markup/html/bootstrap/BootstrapThemeThemeCssReference.java

@@ -5,16 +5,16 @@
 import de.agilecoders.wicket.webjars.request.resource.WebjarsCssResourceReference;
 import org.apache.wicket.markup.head.CssHeaderItem;
 import org.apache.wicket.markup.head.HeaderItem;
+import org.apache.wicket.request.resource.CssResourceReference;
 
 import java.util.List;
 
 /**
  * #### Description
  *
- * The BootstrapThemeThemeCssReference uses the latest version of bootstrap-theme.css from the `org.webjars.bootstrap`
- * dependency which is defined in `pom.xml`. This reference has a dependency to base bootstrap css reference, but there
- * will be no version conflicts between this reference and the `bootstrap.css` from `BootstrapCssReference` because both
- * will are loaded from same jar file.
+ * The BootstrapThemeThemeCssReference uses bootstrap stylesheet with enabled gradient and shadows
+ * to mimic Bootstrap 3 `bootstrap-theme.css` stylesheet. Stylesheet is manually compiled, so
+ * there may be version conflict with future versions.
  *
  * #### Usage
  *
@@ -31,7 +31,7 @@
  *
  * @author Michael Haitz <[email protected]>
  */
-public class BootstrapThemeThemeCssReference extends WebjarsCssResourceReference {
+public class BootstrapThemeThemeCssReference extends CssResourceReference {
     private static final long serialVersionUID = 1L;
 
     /**
@@ -52,7 +52,7 @@ private static final class Holder {
      * Private constructor to prevent instantiation.
      */
     private BootstrapThemeThemeCssReference() {
-        super("/bootstrap/current/css/bootstrap-theme.css");
+        super(BootstrapThemeThemeCssReference.class, "css/bootstrap-theme.css");
     }
 
     @Override