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

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

beat/web/libraries/templates/libraries/edition.html

@@ -192,7 +192,7 @@
 <div class="row">
   <div class="col-sm-10 col-sm-offset-1">
       <label for="source-code">Source code:</label>
-      <textarea id="source-code">{% if library %}{{ library.source_code_file.read }}{% endif %}</textarea>
+      <textarea id="source-code">{% if library %}{{ library.source_code }}{% endif %}</textarea>
       <p class="help">{{ texts.library_code|safe }}</p>
     </div>
 </div>

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

@@ -74,7 +74,7 @@
     {% endwith %}
 
     {% if open_source %}
-    <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>
     {% else %}
     <div class="alert alert-warning">