[algorithms][templates] Use file field property to read content

Algorithms have properties related to their file fields
that does the proper handling for reading their content
use that.

beat/web/algorithms/templates/algorithms/panels/editor.html

@@ -246,7 +246,7 @@
     {% endwith %}{# groups, uses or parameters #}
 
     {% if open_source and not object.is_binary %}
-    <textarea class="form-control" id="code-display">{{ object.source_code_file.read }}</textarea>
+    <textarea class="form-control" id="code-display">{{ object.source_code }}</textarea>
     <p class="help-block">{{ texts.code|safe }}</p>
     {% elif object.is_binary %}
       {% if object.valid %}