* add a selenium test for firefox
* add geckodriver and firefox to travis
* install updated
* install updated
* try out suggested webdriver dependency class
* add more resilience
The version of Maven is not compatible with the plugin "org.codehaus.mojo:flatten-maven-plugin:1.2.5" used in webgoat-container
```
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] Using 'UTF-8' encoding to copy filtered properties files.
[INFO] Copying 1 resource
[INFO] Copying 111 resources
[INFO] The encoding used to copy filtered properties files have not been set. This means that the same encoding will be used to copy filtered properties files as when copying other filtered resources. This might not be what you want! Run your build with --debug to see which files might be affected. Read more at https://maven.apache.org/plugins/maven-resources-plugin/examples/filtering-properties-files.html
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO]
[INFO] webgoat-container ................................. FAILURE [ 18.319 s]
...
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 26.001 s
[INFO] Finished at: 2021-02-06T09:37:58+00:00
[INFO] Final Memory: 49M/162M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.codehaus.mojo:flatten-maven-plugin:1.2.5:flatten (flatten) on project webgoat-container: The plugin org.codehaus.mojo:flatten-maven-plugin:1.2.5 requires Maven version 3.2.5 -> [Help 1]
```
* Remove Raspberry pi Docker file
* Remove old show-source/solution and plan button
* Remove commented out code
* Improve navigation
* Remove underline from navigation and remove margin of left navigation button
* Make arrow a bit smaller so it aligns a bit better with the navigation bar itself
- Output messages more clear
- Replace success to failure (otherwise lesson is marked complete immediately)
- Fix the layout of the lesson (remove whitespace update buttons)
