Make theme able to load custom javascript (#1211)

* Make theme able to load custom javascript
- Look for the field 'customScript' in editorTheme (settings.js)
- Add it to mustach context
- Load list on template side (index.mst)

* Add unit tests for customScripts

* Code review edits : generic behavior for theme.page.[css|scripts]
- Use the same way to share css and javascript files from a theme
- Allow string instead of array for theme.page.scripts
- Remove old customScript field

editor/templates/index.mst

@@ -199,6 +199,9 @@
 <script src="vendor/ace/ext-language_tools.js"></script>
 <script src="{{ asset.red }}"></script>
 <script src="{{ asset.main }}"></script>
+{{# page.scripts }}
+<script src="{{.}}"></script>
+{{/ page.scripts }}
 
 </body>
 </html>