Use FeedEnclosure throughout RSSUtils.

The idea here is to get rid of the intermediate array that gets created for enclosures during RSSUtils::update_rss_feed().

I believe the main risk would be if any plugins with RSSUtils::update_rss_feed()-related hooks that get passed $article (e.g. HOOK_ARTICLE_FILTER) depended upon $article["enclosures"] being an array of arrays; if you think that's likely this can just be closed (or something like ArrayAccess could be added). Outside of the update process it looks like everything just uses Article::_get_enclosures(), which returns an array of modified Idiorm records from ttrss_enclosures.