Adds a `$res` SCSS variable set to the path from the root SCSS file to the `res`
directory.
This is a different base path than previously used in CSS URLs (it goes up 3
directories instead of 2), because Webpack will now be resolving images relative
to the root SCSS file, so the path corresponds to a source tree location,
instead of a path in the build output tree.
Defining this variable has two main goals:
* URLs are a bit easier to read
* The path can be overridden, which is needed for external (riot-web) themes
The "New Recovery Method" dialog would show if either the recovery method had
been changed or removed, but the dialog text didn't make much sense for the
removed case.
This adds a separate dialog customized for the removed case.
Fixes https://github.com/vector-im/riot-web/issues/8046.
J. Ryan Stinnett