Fixed NotificationMailer failing with mentions

[mayaeh]

If you receive a mention with media from a remote instance, the following error may occur.

Job: NotificationMailer#mention
Class: ActionView::Template::Error
Message: No route matches {:action=>"show", :controller=>"media", :id=>#<MediaAttachment id: xxxxx, status_id: xxxxx, file_file_name: “xxxxx.png", file_content_type: "image/png", file_file_size: xxxxx, file_updated_at: "2019-09-04 12:57:35", remote_url: "https://example.com/…”, account_id: xxxxx, created_at: "2019-09-04 12:57:35", updated_at: "2019-09-04 12:57:36", shortcode: nil, type: "image", file_meta: {"original"=>{"width"=>346, "height"=>309, "size"=>"346x309", "aspect"=>1.1197411003236246}, "small"=>{"width"=>346, "height"=>309, "size"=>"346x309", "aspect"=>1.1197411003236246}}, description: nil, scheduled_status_id: nil, blurhash: "U268U2?IogRk_4%MadIos:xtaJIV%Nt6ofR+">}, possible unmatched constraints: [:id]

This happens with accounts that have email notifications enabled for mentions.

This PR fix it.