[Feature] Forms refactoring

This PR concentrates on refactoring forms. The major change is adding a new Twig form extension (copied from GenemuFormBundle, I assumed they will agree as its an open source project too, but I [opened a ticket](https://github.com/genemu/GenemuFormBundle/issues/278) just in case). This extension is only loaded if **GenemuFormBundle** is not loaded in `AppKernel.php` as a fallback.

This extension introduces new twig functions `form_javascript()` and `form_stylesheet()` and a very cool `javascript_prototype` feature, which is now used by `collection` widget. This will automatically add and call javascript for new fields added by collection widget.

> **Note:** the form type useing javascript must use **GenemuFormBundle**'s `javascript_prototype` block to be automagically added by collection widget. Most of **GenemuFormBundle**'s forms do (or I plan soon push a PR to that project to fix the remaining ones).

This PR also adds collapsable behaviour for `collection` **table** widget.

Other minor fixes include:
* fixed delete in `collection.js`
* added `CONTRIBUTING.MD` note
* added suggestion in `composer.json` for **GenemuFormBundle**