templates: add safe filter to html_tags

* otherwise, the HTML tags are escaped and thus not interpreted by the
  browser

invenio_previewer/templates/invenio_previewer/pdfjs.html

@@ -2,6 +2,7 @@
 
   This file is part of Invenio.
   Copyright (C) 2015-2019 CERN.
+  Copyright (C)      2022 TU Wien.
 
   Invenio is free software; you can redistribute it and/or modify it
   under the terms of the MIT License; see LICENSE file for more details.
@@ -10,7 +11,7 @@
 {%- extends config.PREVIEWER_BASE_TEMPLATE %}
 
 {%- block html_tags %}
-  {{ html_tags }}
+  {{ html_tags|safe }}
 {%- endblock %}
 
 {%- block head %}

invenio_previewer/templates/semantic-ui/invenio_previewer/pdfjs.html

@@ -2,6 +2,7 @@
 
   This file is part of Invenio.
   Copyright (C) 2015-2020 CERN.
+  Copyright (C)      2022 TU Wien.
 
   Invenio is free software; you can redistribute it and/or modify it
   under the terms of the MIT License; see LICENSE file for more details.
@@ -10,7 +11,7 @@
 {%- extends config.PREVIEWER_BASE_TEMPLATE %}
 
 {%- block html_tags %}
-  {{ html_tags }}
+  {{ html_tags|safe }}
 {%- endblock %}
 
 {%- block head %}