Added upgrade notice to avoid timeouts

Fixes https://github.com/owncloud/core/issues/10486

Still to be discussed whether it makes sense.

![upgradenotice](https://cloud.githubusercontent.com/assets/277525/3951575/86042c46-26d8-11e4-8306-e61d6f7100da.jpg)


@owncloud/designers @karlitschek @DeepDiver1975 @craigpg