Dashboard greeting names who is signed in

It greets the account by its full name where it has one, and otherwise by
the name it signs in with. The line beneath says who is signed in. The account
is looked up by its local part and matched on the whole address, so a
same-named account on another domain can't answer. The username is kept from
the JMAP session and cleared on sign-out.

src/i18n/en.json

@@ -389,6 +389,7 @@
     "morning": "Good morning",
     "afternoon": "Good afternoon",
     "evening": "Good evening",
-    "subtitle": "Here's how your mail server is doing."
+    "subtitle": "Here's how your mail server is doing.",
+    "signedInAs": "Signed in as {{username}}"
   }
 }