Remove triple braces for i18n blocks in 404 and anno edit templates

The intent of the triple braces was to prevent escaping, but
handlebars-i18next already disables escaping. After a `gulp clean` I
found out that gulp-handlebars was actually tripping over this
notation.

frontend/src/notfound404/notfound-template.hbs

@@ -4,9 +4,9 @@
         alt="{{#i18n 'not_found_alt'}}404 Not Found{{/i18n}}"
     >
 </figure>
-<p>{{{#i18n 'not_found_text'}}}
+<p>{{#i18n 'not_found_text'}}
     Oops, this page doesn't seem to exist.
     You can <a onclick="history.back()">go back</a>,
     visit <a href="/landing">Landing</a> or <a href="/search">Search</a>,
     or report a problem to the maintainers.
-{{{/i18n}}}</p>
+{{/i18n}}</p>

frontend/src/panel-annotation/annotation-edit-template.hbs

@@ -1,9 +1,9 @@
 <div class="panel-header">
     <header>
         <p class="title is-3">{{#i18n 'edit_annotation'}}Edit Annotation{{/i18n}}</p>
-        <p class="subtitle is-7">{{{#i18n 'annotation_info'}}}(Find more info on what and how to annotate in
+        <p class="subtitle is-7">{{#i18n 'annotation_info'}}(Find more info on what and how to annotate in
             <a class="is-link" href="http://dhstatic.hum.uu.nl/readit/AnnotationGuide.pdf" target="_blank"> the
-                Annotation Guide</a>{{{/i18n}}})
+                Annotation Guide</a>{{/i18n}})
         </p>
     </header>
 </div>