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/upload_media_asset.rb

@@ -41,6 +41,21 @@ class UploadMediaAsset < ApplicationRecord
     expired.update_all(status: :failed, error: "Stuck processing for more than 4 hours")
   end
 
+  def self.is_matches(value)
+    case value.downcase
+    when *UploadMediaAsset.statuses.keys
+      where(status: value)
+    when *MediaAsset::FILE_TYPES
+      attribute_matches(value, :file_ext, :enum)
+    else
+      none
+    end
+  end
+
+  def self.exif_matches(string)
+    merge(MediaAsset.exif_matches(string))
+  end
+
   def self.search(params, current_user)
     q = search_attributes(params, [:id, :created_at, :updated_at, :status, :source_url, :page_url, :error, :upload, :media_asset, :post], current_user: current_user)