MDL-59884 core: add new overlay_loading template

lib/templates/overlay_loading.mustache

@@ -0,0 +1,38 @@
+{{!
+    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/>.
+}}
+{{!
+    @template core/overlay_loading
+
+    Moodle overlay loading template.
+
+    The purpose of this template is to render a loading animation using a overlay effect.
+
+    Classes required for JS:
+    * none
+
+    Data attributes required for JS:
+    * none
+
+    Context variables required for this template:
+    *
+
+    Example context (json):
+    {}
+}}
+<span class="overlay-icon-container hidden" data-region="overlay-icon-container">
+    {{> core/loading }}
+</span>