[doc] Adding 2.4 release and docs
[site] Pointing to 2.4 instead of 2.3
[site] Added mailing list subscribe field
[site] Download gives more options now
[site] Added HBase app and video gallery
[site] Fixes css cache
[doc] 2.5 release
[site] Added Hue 2.5 video to the gallery
[site] Added HCatalog video and scroll buttons to video list
[site] Added HA video
[site] Changed download links and added GA tracking of downloads