New plugin: Verify new versions from GitHub

Thanks to @adrianbj in 441e7f0.

plugins/version-github.php

@@ -0,0 +1,34 @@
+<?php
+
+/** Verify new versions from GitHub
+* @link https://www.adminer.org/plugins/#use
+* @author Jakub Vrana, https://www.vrana.cz/
+* @license https://www.apache.org/licenses/LICENSE-2.0 Apache License, Version 2.0
+* @license https://www.gnu.org/licenses/gpl-2.0.html GNU General Public License, version 2 (one or other)
+*/
+class AdminerVersionGithub {
+
+	function head($dark = null) {
+		?>
+<script <?php echo Adminer\nonce(); ?>>
+verifyVersion = (current, url, token) => {
+	// dummy value to prevent repeated verifications after AJAX failure
+	cookie('adminer_version=0', 1);
+	ajax('https://api.github.com/repos/vrana/adminer/releases/latest', request => {
+		const response = JSON.parse(request.responseText);
+		const version = response.tag_name.replace(/^v/, '');
+		// we don't save to adminer.version because the response is not signed; also GitHub can handle our volume of requests
+		// we don't display the version here because we don't have version_compare(); design.inc.php will display it on the next load
+		cookie('adminer_version=' + version, 1);
+	}, null, null);
+};
+</script>
+<?php
+	}
+
+	function csp() {
+		$csp = Adminer\csp();
+		$csp[0]["connect-src"] .= " https://api.github.com/repos/vrana/adminer/releases/latest";
+		return $csp;
+	}
+}