Add ability to hide sender's email address (hide-sender-address setting)

The configuration setting is called `Hide*` instead of `Show*`, because
it's backward compatible with existing configuration settings.

This is useful for when you setup an email forwarding inbox and you're
always sending to it through the same email address. In that case, you
don't need to see the email address in each Matrix message.

In the future, another similar `bool` setting (`hide-subject`) will land,
which controls whether the email's subject is shown in the final message
or not. That setting can make use of most of the same setup (all of
`handleBooleanConfigurationKey`).

bot/data.go

@@ -14,8 +14,9 @@ var migrations = []string{}
 
 // settings of a room
 type settings struct {
-	Mailbox string
-	Owner   id.UserID
+	Mailbox           string
+	Owner             id.UserID
+	HideSenderAddress bool
 }
 
 // Allowed checks if change is allowed