firmware: add a message for announcing major firmware changes

src/opnsense/firmware-message

+<p>Hello trusty friend,</p>
+<p>As with all things in life, the 15.7 series of OPNsense is nearing its
+inevitable end. As such it will not receive any more updates, but the
+upgrade to the new 16.1 series is seamless, except for the following
+points:</p>
+<p><ul><li>The FreeBSD version changes from 10.1 to 10.2, mainly for driver
+updates and general sanity. <strong>If you're running Hyper-V, your installed
+disk will change from <em>/dev/ada0</em> to <em>/dev/da0</em> and the system
+will not boot as a consequence</strong>. You can fix this by manually editing
+<em>/etc/fstab</em>. A reinstall using the import configuration and quick/easy
+install will work just as well.</li>
+<li>The captive portal implementation has been completely rewritten.
+<strong>Your old setup won't be converted and thus will not come back
+up</strong>. Reconfiguring the captive portal, however, will be relatively
+easy.</li></ul></p>
+<p>Please heed these points carefully before upgrading. Backup your configs,
+preview the new version via the live CD or in a virtual machine. Create
+snapshots. If all else fails, report back
+<a href="https://forum.opnsense.org/" target="_blank">in the forums</a> for
+assistance. You don't have to do this on your own. :)</p>
+<p><em>Brave Badger</em>, you've served us well.</p>
+<p>See you on the other side,<br>Your OPNsense team</p>

src/opnsense/mvc/app/views/OPNsense/Core/firmware.volt

@@ -203,6 +203,10 @@ POSSIBILITY OF SUCH DAMAGE.
 
 <div class="container-fluid">
     <div class="row">
+<?php   $message = @file_get_contents('/usr/local/opnsense/firmware-message');
+        if (!empty($message)): ?>
+        <div class="alert alert-warning" role="alert"><?= $message ?></div>
+<?php   endif; ?>
         <div class="alert alert-info" role="alert" style="min-height: 65px;">
             <button class='btn btn-primary pull-right' id="upgrade" style="display:none"><i id="upgrade_progress" class=""></i> {{ lang._('Upgrade now') }}</button>
             <button class='btn btn-default pull-right' id="checkupdate" style="margin-right: 8px;"><i id="checkupdate_progress" class=""></i> {{ lang._('Fetch updates')}}</button>