Thumbnails from server

Preview thumbnail from server for images that are not downloaded.
This uses newly created rest api for thumbnails (https://github.com/owncloud/core/pull/11009).
We should think in refactoring a bit this to add a "Thumbnail" resource in owncloud-library, but maybe we should merge this first.

On the while, some low level details:
 -  ```java Uri.encode(file.getRemotePath(), "/") ``` does the same job than ```java URLEncoder.encode(file.getRemotePath(), "UTF-8").replaceAll("%2F", "/") ```, but shorter.
-  Instead of hardcoding 100 x 100, define constants like THUMB_WIDTH, THUMB_HEIGHT ; or even better, we can try to request the lowest resolution appropiate to get 32dp thumbnails, depending on the screen density.