userpage: implement login message card

Shares code with custom emails, so most related functions have had a
%s/Email/Message/g. Press the edit button on the user page setting to
add a message.

html/header.html

@@ -1,4 +1,4 @@
-<meta charset="utf-8">
+<meta charset="UTF-8"/>
 <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
 <meta name="Description" content="jfa-go, a better way to manage Jellyfin users.">
 <meta name="color-scheme" content="dark light">