Design fixes

Various design detail fixes. Please check @owncloud/designers 

* reduce z-index of settings popup, fix #4212 cc @stonie08
* remove ugly inset shadow from header
* remove unused and never to be used pagination
* remove table layout for background jobs
* wrap code output in error messages, fix #4231 cc @DeepDiver1975 
* change long couldn't be determined string to question mark again