add .disabled class so it can be used for simple a buttons as well

ref #2576 
@DeepDiver1975 this will make the new and upload buttons work with disabled styles, you just need to give them class="disabled" – can you update that part with https://github.com/owncloud/core/pull/2576#issuecomment-16064656 ? We also need to backport both this and that then.

Review please @Raydiation.