Remove backup/restore and factory reset; add APRS/POCSAG custom settings

- Remove admin/config_backup.php (backup/restore page) and all nav
  links to it across the admin pages and the expert-mode shared header.
- Remove factory reset: the full-device version in configure.php
  (JS confirm, POST handler that reset three git repos + wiped /etc
  config, and the hidden form) and the CSS-only version in
  expert/edit_dashboard.php.
- Remove the PiStar-Remote and BM API full editors
  (admin/expert/fulledit_pistar-remote.php, fulledit_bmapikey.php)
  and their links from the expert-mode menu.
- Add a "custom" option to the APRS server dropdown in configure.php
  so operators can type their own server + port instead of picking
  from APRSHosts.txt; wires the port through to ircDDBGateway,
  YSFGateway/YSF2DMR/YSF2NXDN/YSF2P25, and APRSGateway (previously
  only the hostname was ever configurable, port was hardcoded 14580).
- Extend the POCSAG frequency band validator (functions.js) to also
  treat 150-160MHz as a valid/green range.

admin/live_modem_log.php

@@ -94,7 +94,6 @@ if ($_SERVER["PHP_SELF"] == "/admin/live_modem_log.php") {
     <a href="/" style="color: #ffffff;"><?php echo $lang['dashboard'];?></a> |
     <a href="/admin/" style="color: #ffffff;"><?php echo $lang['admin'];?></a> |
     <a href="/admin/power.php" style="color: #ffffff;"><?php echo $lang['power'];?></a> |
-    <a href="/admin/config_backup.php" style="color: #ffffff;"><?php echo $lang['backup_restore'];?></a> |
     <a href="/admin/configure.php" style="color: #ffffff;"><?php echo $lang['configuration'];?></a>
   </p>
   </div>