[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.

docs/gallery-dl.conf

@@ -93,6 +93,7 @@
         },
         "deviantart":
         {
+            "refresh-token": null,
             "flat": true,
             "mature": true,
             "original": true