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`.
Had to make various other changes due to incompatibilities.
- Use the new `lit` package instead of `lit-html` or `lit-element`
- Drop `haunted` since it breaks the rules by specifying `type: module`
but then doesn't import with file extensions
- Use Sass Dart instead of node-sass (fixes#2445)
- Upgrade Karma
