Styling of Homepage

I made some minor UI changes to the labels home page:

* aligned the images and titles by defining a maxwidth for the image container
* hide the close button on the "Choose your community" alert text - which makes no sense at this point
* reduce (empty) header height, so content comes earlier

![preview](https://f.cloud.github.com/assets/147175/62126/d88e18aa-5cdb-11e2-851e-add1147eb60b.jpg)

I also want to improve the admin UI, e.g.: selecting the right user as organizer of an event, is a little bit tedious, maybe also using a better date/time-picker, if that is ok.