Add ability to bulk delete files in "My private files" repository

Behat and unit tests provided

Test with boost and clean.

Prerequistes: 

1. Login as any user. 
2. Go to 'Private files'
3. Upload any file, name it "F1"
4. Upload any file, name it "F2"
5. Upload any file, name it "F3"
6. Create a folder "Folder1"
7. In "Folder1", upload any file, name it "F4"
8. Save Changes

Testing:

1. Individual delete
   1. Login as user from above
   2. Go to 'Private files'
   3. Select "Display folder with details" (middle icon on the right)
   4. Confirm table of files with checkboxes in the first cell for each row
   5. Enable F1's checkbox
   6. Click on 'Delete selected' button (identified with a bin icon)
   7. Confirm F1 is removed from table
   8. Save changes
   9. Confirm F1 is still not shown 
2. Multiple delete
   1. Login as user from above
   2. Go to 'Private files'
   3. Select "Display folder with details" 
   4. Confirm table of files with checkboxes in the first cell for each row
   5. Enable F2's checkbox
   6. Enable F3's checkbox
   7. Enable Folder1's checkbox
   8. Click on 'Delete selected' button (identified with a bin icon)
   9. Confirm Folder 1, F2 and F3 are removed from table
   10. Save changes
   11. Confirm Folder 1, F2 and F3 are still not shown