Convert from BoltDB to Maildir storage for emails

DESIGN.md

@@ -40,19 +40,9 @@ following, though:
     * `Admin`
     * `Wildcard`
     * `PasswordHash`
-  * `Message`
-    * `ID`
-    * `Username`
-    * `Mailbox`
-    * `Header`
-    * `Body`
 
 I don't seem to be able to get accounts for a list of usernames very easily, or
 indexed-ly.
 
-The message body is being stored inefficiently (json []byte, so base64-encoded)
-
-Message username and mailbox are indexed, but is that good enough? Perhaps we
-should have a bucket per mailbox or something?
-
-TODO: rewrite this (and the code) to make Maildir into a thing
+Messages are stored in a traditional Maildir structure structure following a
+`<home>/mailboxes/<domain>/<username>` hierarchy.