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/media_asset.rb

@@ -204,7 +204,30 @@ class MediaAsset < ApplicationRecord
   concerning :SearchMethods do
     class_methods do
       def ai_tags_match(tag_string, score_range: (50..))
-        AITagQuery.search(tag_string, relation: self, score_range: score_range)
+        MediaAssetQuery.search(tag_string, relation: self, score_range: score_range)
+      end
+
+      def is_matches(value)
+        case value.downcase
+        when *MediaAsset.statuses.keys
+          where(status: value)
+        when *FILE_TYPES
+          attribute_matches(value, :file_ext, :enum)
+        else
+          none
+        end
+      end
+
+      def exif_matches(string)
+        # string = File:ColorComponents=3
+        if string.include?("=")
+          key, value = string.split(/=/, 2)
+          hash = { key => value }
+          joins(:media_metadata).where_json_contains("media_metadata.metadata", hash)
+        # string = File:ColorComponents
+        else
+          joins(:media_metadata).where_json_has_key("media_metadata.metadata", string)
+        end
       end
 
       def search(params, current_user)