Various Website improvements

Use an homogeneous layout (section titles and content) for:
- /documentation/whatsnew
- /downloads/jbosstools, /downloads/jbosstools/luna, etc.
Setting a green background on the 'hero' divs
Alignin buttons on the left
Using flex layout to have same-height sections everywhere
Adding a border around the content in /blog, /downloads/.. and /whatsnew
Adding 3 incoming events
Adding a navigation menu in the /blog part of the site
Adding a link to the latest beta versions in /downloads