Make d2h-d-none override visibility: fix broken hiding of viewed files

[jacekkopecky]

Fixes a regression: toggling Viewed no longer hides the patch for the file.

Steps to reproduce:

1. *** show a diff (e.g. at https://jsbin.com/salayecelu/edit?html,output )
2. *** click "Viewed"
3. *** observe that the file is still shown

Expected Results:

The file should hide when "Viewed" is toggled on.

I believe the problem was introduced in b164b511d80edf25ca942b50dc4911b12257b7ee

Since .d2h-d-none is the first rule in the file, other rules with the same specificity override it, e.g. line 102 in .d2h-files-diff. This means hiding no longer works. This commit fixes that; alternatively, the .d2h-d-none rule should be moved to the end of the file if you don't like !important.