Fix #3615: Unsupported video codecs.

Don't allow uploading videos with unsupported video codecs.

The only video codecs we allow for MP4 files are H.264 and VP9. Other
codecs, including H.265 (aka HEVC), MPEG-4 part 2, and AV1, are
disallowed because they're not universally supported by browsers.
Firefox doesn't support H.265 or MPEG-4 part 2, and Safari doesn't
support AV1.

Additionally, don't allow videos with multiple video tracks, multiple
audio tracks, or no video tracks. Multiple video and audio tracks are
disallowed because they're rare and for moderation purposes, we don't
want people hiding content in extra tracks.

These restrictions really only apply to MP4 videos, since WebM files
don't support multiple video or audio tracks and only support a limited
number of codecs (VP8 and VP9 for videos, Vorbis and Opus for audio).

There are currently 22 posts with unsupported video codecs:

* https://danbooru.donmai.us/posts?tags=video+is:mp4+-exif:Track1:CompressorID=avc1+-exif:Track2:CompressorID=avc1+-exif:Track1:CompressorID=vp09+-exif:Track2:CompressorID=vp09 # AVC1 is H.264

There is one post that has multiple audio tracks:

* https://danbooru.donmai.us/posts/2382057

test/components/post_preview_component_test.rb

@@ -31,7 +31,7 @@ class PostPreviewComponentTest < ViewComponent::TestCase
 
     context "for a video post with sound" do
       should "render" do
-        @post = create(:post_with_file, tag_string: "sound", filename: "test-audio.mp4").reload
+        @post = create(:post_with_file, tag_string: "sound", filename: "mp4/test-audio.mp4").reload
         node = render_preview(@post, current_user: User.anonymous)
 
         assert_equal(post_path(@post), node.css("article a").attr("href").value)