implement a catchall address for a domain

by specifying a "destination" in an account that is just "@" followed by the
domain, e.g. "@example.org". messages are only delivered to the catchall
address when no regular destination matches (taking the per-domain
catchall-separator and case-sensisitivity into account).

for issue #18

http/account.html

@@ -273,6 +273,7 @@ const index = async () => {
 			Object.entries(destinations).sort().map(t =>
 				dom.li(
 					dom.a(t[0], attr({href: '#destinations/'+t[0]})),
+					t[0].startsWith('@') ? ' (catchall)' : [],
 				),
 			),
 		),