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

rest/testutils_test.go

@@ -17,7 +17,7 @@ import (
 
 type InputMessageData struct {
 	Mailbox, ID, From, Subject string
-	To                         string
+	To                         []string
 	Date                       time.Time
 	Size                       int
 	Header                     mail.Header
@@ -81,8 +81,10 @@ func (d *InputMessageData) CompareToJSONHeaderMap(json interface{}) (errors []st
 		if msg, ok := isJSONStringEqual(fromKey, d.From, m[fromKey]); !ok {
 			errors = append(errors, msg)
 		}
-		if msg, ok := isJSONStringEqual(toKey, d.To, m[toKey]); !ok {
-			errors = append(errors, msg)
+		for i, inputTo := range d.To {
+			if msg, ok := isJSONStringEqual(toKey, inputTo, m[toKey].([]interface{})[i]); !ok {
+				errors = append(errors, msg)
+			}
 		}
 		if msg, ok := isJSONStringEqual(subjectKey, d.Subject, m[subjectKey]); !ok {
 			errors = append(errors, msg)