[mastodon] support '/@USER/following' URLs

Previously, only '/users/USER/following' got matched.

gallery_dl/extractor/mastodon.py

@@ -131,8 +131,8 @@ class MastodonBookmarkExtractor(MastodonExtractor):
 class MastodonFollowingExtractor(MastodonExtractor):
     """Extractor for followed mastodon users"""
     subcategory = "following"
-    pattern = BASE_PATTERN + r"/users/([^/?#]+)/following"
-    example = "https://mastodon.social/users/USER/following"
+    pattern = BASE_PATTERN + r"/(?:@|users/)([^/?#]+)/following"
+    example = "https://mastodon.social/@USER/following"
 
     def items(self):
         api = MastodonAPI(self)

test/results/mastodonsocial.py

@@ -70,6 +70,12 @@ __tests__ = (
     "#extractor": False,
 },
 
+{
+    "#url"     : "https://mastodon.social/@0x4f/following",
+    "#category": ("mastodon", "mastodon.social", "following"),
+    "#class"   : mastodon.MastodonFollowingExtractor,
+},
+
 {
     "#url"     : "https://mastodon.social/users/id:10843/following",
     "#category": ("mastodon", "mastodon.social", "following"),