This allows for more flexibility in configuring which media URLs will
automatically render and which media URLs may be manually rendered by
the user (via the message actions dropdown).
For example, suppose you want to automatically render all media URLs
from https://xmpp.org, but still allow other media (which won't render
by default) to be rendered manually by the user (by clicking the "Show
URL previews" message dropdown action).
In this case, you set `render_media` to `['xmpp.org']` and
`allowed_image_domains` to `null` or `undefined`.
Or if you want to automatically render images from xmpp.org, and
restrict the domains users might manually click to render, you can add
those extra domains to `allowed_image_domains`.
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
