MDL-61958 rss: Create core_rss lang file and move rss string

At the moment will only be copied one of the rss strings ('rss').
The issue MDL-61997 has been created to move the rest of the core_rss lang
strings from moodle.php to rss.php.

AMOS BEGIN
 CPY [rss,core],[rss,core_rss]
AMOS END

lang/en/rss.php

@@ -0,0 +1,27 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * RSS language file.
+ *
+ * @package    core_rss
+ * @copyright  2018 Sara Arjona <sara@moodle.com>
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+defined('MOODLE_INTERNAL') || die();
+
+$string['rss'] = 'RSS';

rss/renderer.php

@@ -57,7 +57,7 @@ public function user_rss_token_box($token) {
         $stroperation = get_string('operation', 'webservice');
         $strtoken = get_string('key', 'webservice');
 
-        $return = $OUTPUT->heading(get_string('rss'), 3, 'main', true);
+        $return = $OUTPUT->heading(get_string('rss', 'rss'), 3, 'main', true);
         $return .= $OUTPUT->box_start('generalbox webservicestokenui');
 
         $return .= get_string('rsskeyshelp');