Enhancement: Support for spaces

Spaces are now supported in oCIS accounts. A new tab has been added, which allows to list and
browse through all the available spaces for the current account. The supported operations
for files in spaces are: download, upload, remove, rename, create folder, copy and move. The
documents provider has been adapted as well to be able to browse through spaces and perform
the operations already mentioned.

https://github.com/owncloud/android/pull/3851
