[deviantart] add support for OAuth2 authentication

Some user galleries [*] require you to be either logged in or
authenticated via OAuth2 to access their deviations.

[*] e.g. https://polinaegorussia.deviantart.com/gallery/

--------------

known issue:
A deviantart 'refresh_token' can only be used once and gets updated
whenever it is used to request a new 'access_token', so storing its
initial value in a config file and reusing it again and again is not
possible.

test/test_extractors.py

@@ -82,8 +82,7 @@ skip = [
     "exhentai", "kissmanga", "mangafox", "dynastyscans", "nijie",
     "archivedmoe", "archiveofsins", "thebarchive",
     # temporary issues
-    "mangareader",
-    "mangapanda",
+    "hbrowse",
 ]
 # enable selective testing for direct calls
 if __name__ == '__main__' and len(sys.argv) > 1: