Switch to storing To addresses as a slice

- Changes on-disk storage format
- Changes JSON API
- To and From values are now parsed/formatted by Go's mail.ParseAddress
  function
- Fixed bug in list-entry-template, was not escaping HTML characters
- Updated tests

themes/bootstrap/templates/mailbox/_show.html

@@ -53,7 +53,12 @@
       <dt>From:</dt>
       <dd>{{.message.From}}</dd>
       <dt>To:</dt>
-      <dd>{{.message.To}}</dd>
+      <dd>
+      {{range $i, $addr := .message.To}}
+        {{- if $i}},{{end}}
+        {{$addr -}}
+      {{end}}
+      </dd>
       <dt>Date:</dt>
       <dd>{{.message.Date}}</dd>
       <dt>Subject:</dt>