even if the global configuration is to disallow it.
* When parsing, include all media URLs, not just the ones from allowed domains.
That makes it possible to change allowed domains on-the-fly,
while still allowing media in individual messages to be shown manually
(via the message actions dropdown).
* Merge `embed_audio`, `embed_video` and `show_images_inline` into `render_media`
* Create new config settings for allowable domains for images, video and audio
* Check the URL domain against a whitelist for the message actions dropdown
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