Update installation pages

Use nav pills for progress, add some icons and use bootstrap for updating/upgrading pages.

Changes:
* Button on first step now says "I agree, continue". This makes more sense for the license.
* Use nav pills for viewing progress
* Use bootstrap thumbnails in step 4
* Add icons for continue & back buttons, as well as for completed steps
* Footer now also has margin-top so it doesn't stick to the content that much
* I also changed paragraphs to h4, I think this looks nicer and also makes the text stand out a little bit more
* Added button for deleting the installation directory