uploads: allow searching uploads and media assets by metatag.

Allow searching the /uploads and /media_assets pages by the following metatags:

* id:
* md5:
* width:
* height:
* duration:
* mpixels:
* ratio:
* filesize:
* filetype:
* date:
* age:
* status:<processing|active|deleted|expunged|failed> (for /media_assets)
* status:<pending|processing|active|failed> (for /uploads)
* is:<filetype>, is:<status>
* exif:

Examples:

* https://betabooru.donmai.us/media_assets?search[ai_tags_match]=filetype:png
* https://betabooru.donmai.us/uploads?search[ai_tags_match]=filetype:png

Note that in /uploads search, the id:, date:, and age: metatags refer to the upload media asset, not
the upload itself.

Note also that uploads may contain multiple assets, so for example searching uploads by
`filetype:png` will return all uploads containing at least one PNG file, even if they contain other
non-PNG files.

app/models/post.rb

@@ -1284,17 +1284,7 @@ class Post < ApplicationRecord
       end
 
       def exif_matches(string)
-        # string = exif:File:ColorComponents=3
-        if string.include?("=")
-          key, value = string.split(/=/, 2)
-          hash = { key => value }
-          metadata = MediaMetadata.joins(:media_asset).where_json_contains(:metadata, hash)
-        # string = exif:File:ColorComponents
-        else
-          metadata = MediaMetadata.joins(:media_asset).where_json_has_key(:metadata, string)
-        end
-
-        where(md5: metadata.select(:md5))
+        where(md5: MediaAsset.exif_matches(string).select(:md5))
       end
 
       def ai_tags_include(value, default_confidence: ">=50")