refactor: extract frontpage to template (#3130)

Also introduce usage of Response object

templates/frontpage.html.php

@@ -0,0 +1,62 @@
+<script src="static/rss-bridge.js"></script>
+<script>
+    document.addEventListener('DOMContentLoaded', rssbridge_toggle_bridge);
+    document.addEventListener('DOMContentLoaded', rssbridge_list_search);
+</script>
+
+<?php if (Debug::isEnabled()): ?>
+    <?php if (!Debug::isSecure()): ?>
+        <section class="critical-warning">
+            Warning : Debug mode is active from any location,
+            make sure only you can access RSS-Bridge.
+        </section>
+    <?php else: ?>
+        <section class="warning">
+            Warning : Debug mode is active from your IP address,
+            your requests will bypass the cache.
+        </section>
+    <?php endif; ?>
+<?php endif; ?>
+
+<section class="searchbar">
+    <h3>Search</h3>
+    <input
+        type="text"
+        name="searchfield"
+        id="searchfield"
+        placeholder="Insert URL or bridge name"
+        onchange="rssbridge_list_search()"
+        onkeyup="rssbridge_list_search()"
+        value=""
+    >
+</section>
+
+<?= raw($bridges) ?>
+
+<section class="footer">
+    <a href="https://github.com/rss-bridge/rss-bridge">RSS-Bridge ~ Public Domain</a><br>
+    <p class="version"><?= e(Configuration::getVersion()) ?></p>
+
+    <?= $active_bridges ?>/<?= $total_bridges ?> active bridges.<br>
+
+    <?php if ($active_bridges !== $total_bridges): ?>
+        <?php if ($show_inactive): ?>
+            <a href="?show_inactive=0">
+                <button class="small">Hide inactive bridges</button>
+            </a>
+            <br>
+        <?php else: ?>
+            <a href="?show_inactive=1">
+                <button class="small">Show inactive bridges</button>
+            </a>
+            <br>
+        <?php endif; ?>
+    <?php endif; ?>
+
+    <?php if ($admin_email): ?>
+        <span>
+            You may email the administrator of this RSS-Bridge instance at
+            <a href="mailto:<?= e($admin_email) ?>"><?= e($admin_email) ?></a>
+        </span>
+    <?php endif; ?>
+</section>