aliases: Support '*' as the destination user

This patch implements support for aliases that contain '*' as the
destination user.

In that case, we replace it with the original user.

For example, `*: *@pond` will redirect `lilly@domain` to `lilly@pond`.

This is experimental for now, and marked as such in the documentation.
The semantics can be subtle, so we may need to adjust them later.

.mkdocs.yml

@@ -13,6 +13,7 @@ markdown_extensions:
   - codehilite:
       guess_lang: false
   - attr_list
+  - admonition
 
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