Bug fixes for partial views

closes #1203
- Update express-hbs module to the new version (0.5.2)
- Use two instance of hbs one for the theme and an other for the admin
- Template helpers are register as partial view
- Partial views of the theme are reload when the theme changed

Remove clear partial cache in handlebars

This code will be move in `express-hbs`.
This doesn't cause a problem to remove this line but it is not clean.

Remove unused hbs instance

Resolve conflict
Merge pull request #1606 from Gotvitch/issue1203

Bug fixes for partial views