Add background-size: cover; to background image

[Jeremy Hopkins]

Add setting to background image for background-size: cover; to allow large bg image to be used instead of tile

[Alexander Goryntsev]

I think it makes sense to make it as it was claimed to be from the beginning: limit the showing of the background image to the frontpage only. The first reason is that merely the frontpage serves marketing purposes and should look attractive; other pages are for teaching and should focus more on the content. Another reason is that pages other than front one might have complicated layout and a sitewide background image could raise a lot of consequent styling issues. I've played with solid colors for #region-main and see what's coming :)

But the frontpage only we can make clean and nice having quite modern attractive look with a background image.