imgur: download videos as mp4 instead of gif

Some imgur URLS have the extension ".gifv" and show up as a gif,
even though they're actually supposed to be mp4 videos. Imgur
serves all videos/gifs as both .gif and .mp4. The image dict has
a key "prefer_video" to distinguish the two. This commit
overrides the .gif extension if "prefer_video" is true to ensure
we download the submission as originally intended.

tests/site_downloaders/test_imgur.py

@@ -111,7 +111,7 @@ def test_imgur_extension_validation_bad(test_extension: str):
     ),
     (
         'https://imgur.com/gallery/IjJJdlC',
-        ('7227d4312a9779b74302724a0cfa9081',),
+        ('740b006cf9ec9d6f734b6e8f5130bdab',),
     ),
     (
         'https://imgur.com/a/dcc84Gt',
@@ -142,6 +142,14 @@ def test_imgur_extension_validation_bad(test_extension: str):
         'https://imgur.com/ubYwpbk.GIFV',
         ('d4a774aac1667783f9ed3a1bd02fac0c',),
     ),
+    (
+        'https://i.imgur.com/j1CNCZY.gifv',
+        ('58e7e6d972058c18b7ecde910ca147e3',),
+    ),
+    (
+        'https://i.imgur.com/uTvtQsw.gifv',
+        ('46c86533aa60fc0e09f2a758513e3ac2',),
+    ),
 ))
 def test_find_resources(test_url: str, expected_hashes: list[str]):
     mock_download = Mock()