Add Clear files locally option to the side menu, make possible to view the largest files

When the number of locally saved files on the device grows the device storage may be exhausted. At the same time, it is not so easy to find out which files to delete locally. Additionally, under certain security issues, a user may be interested in cleaning all locally saved files from the device.

I suggest to add the following functions to the existing UI:
1. to add the button `Clear files locally` which will lead to the deletion of all locally saved files;
2. to make it possible to show the largest files stored on the device which may be furthermore removed from it thus cleaning local device storage for saving other files.

This will significantly improve UI with a local cloud by adding the functions that exist in native clients of the major public clouds.