fe_comment: recipients are the FE users of the feedback group

- The selectable recipients are no longer a hand-maintained setting but
  every active frontend user of the feedback group (feUserGroup) with a
  valid e-mail address; the id is the fe_users uid, so a stored selection
  survives users joining or leaving. The `recipients` setting is marked
  unused
- Each recipient in the state payload carries label, email and selected
  (the stored choice), for a checkbox list with "select all" above the
  submit button
- submit accepts the checked ids directly ({"recipients": [uids]}),
  validates them against the group, stores and uses them, and reports
  the addresses used as sentTo; no selection falls back to mailTo
- migrations/check_fecomment_user.php: all summaries and items of one
  user incl. deleted ones, for diagnosis

public/_frontend/index.html

@@ -23,8 +23,8 @@
     <meta name="twitter:description" content="Enterprise IPTV, video streaming, and digital signage solutions." />
     <link rel="icon" type="image/svg+xml" href="/fileadmin/icons/favicon.ico" />
     <title>VITEC</title>
-    <script type="module" crossorigin src="/_frontend/assets/index-B1plOsdz.js"></script>
-    <link rel="stylesheet" crossorigin href="/_frontend/assets/index-BADFa6PF.css">
+    <script type="module" crossorigin src="/_frontend/assets/index-CU_W4-il.js"></script>
+    <link rel="stylesheet" crossorigin href="/_frontend/assets/index-D94-Mm7O.css">
   </head>
   <body>
     <div id="page"></div>␍