Fix #4990: Allow admins to delete uploads.

Allow admins to delete media asset files.

This only deletes the image file itself, not the upload or media asset record. The upload will still
be in the user's upload list, but the image will be gone. The media asset page will still exist, but
it will only show the file's metadata, not the image itself. We don't delete the metadata so we have
a record of what the file's MD5 was and who uploaded it, to prevent the file from being uploaded
again and to take action against the user if necessary.

app/models/media_asset.rb

@@ -392,17 +392,23 @@ class MediaAsset < ApplicationRecord
       end
     end
 
-    def expunge!
-      delete_files!
-      update!(status: :expunged)
+    def expunge!(current_user, log: true)
+      with_lock do
+        delete_files!
+        update!(status: :expunged)
+        ModAction.log("expunged media asset ##{id} (md5=#{md5})", :media_asset_expunge, subject: self, user: current_user) if log
+      end
     rescue
       update!(status: :failed)
       raise
     end
 
-    def trash!
-      variants.each(&:trash_file!)
-      update!(status: :deleted)
+    def trash!(current_user, log: true)
+      with_lock do
+        variants.each(&:trash_file!)
+        update!(status: :deleted)
+        ModAction.log("deleted media asset ##{id} (md5=#{md5})", :media_asset_delete, subject: self, user: current_user) if log
+      end
     rescue
       update!(status: :failed)
       raise

app/models/mod_action.rb

@@ -42,6 +42,8 @@ class ModAction < ApplicationRecord
     post_vote_undelete: 233,
     pool_delete: 62,
     pool_undelete: 63,
+    media_asset_delete: 72,
+    media_asset_expunge: 76,
     artist_ban: 184,
     artist_unban: 185,
     comment_update: 81,

app/models/post.rb

@@ -780,7 +780,7 @@ class Post < ApplicationRecord
           decrement_tag_post_counts
           remove_from_all_pools
           remove_from_fav_groups
-          media_asset.trash!
+          media_asset.trash!(current_user, log: false)
           destroy
           update_parent_on_destroy
         end