Merge: Use the DOM parser in the RSS Downloader contrib

`rss_downloader` now uses the DOM XML parser instead of regex to read RSS files. It is also easier to customize its behavior from the config class.

This PR also adds a few services to the DOM parser and fix a few bugs.

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This PR is based on #1448 with fixed line endings.

Pull-Request: #1545
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>