Add the wildcard field to accounts

DESIGN.md

@@ -36,4 +36,5 @@ following, though:
   * `Account`
     * `Username`
     * `Admin`
+    * `Wildcard`
     * `PasswordHash`

internal/store/account.go

@@ -25,6 +25,10 @@ type Account struct {
 	// is an admin, but others may be set up too if desired.
 	Admin bool
 
+	// Wildcard users are last-resort destinations for incoming mail. A domain
+	// should only have one of them.
+	Wildcard bool `storm:"index"`
+
 	// As generated by HashPassword
 	PasswordHash string
 }