Enhancement: Implement spaces front page

Each space can now be entered from within the spaces list. The space front page will then display all the space contents, plus an image and a readme file if set. Basic actions like uploading files, creating folders, renaming resources, and more. were also implemented in the course of this.

https://github.com/owncloud/web/pull/6287
https://github.com/owncloud/web/issues/6271
