Implement modal-based front-end in Consultation items management UI

[danguita]

Connects to #92.

What does this PR do?

This PR comes as part of the batch of UI enhancements that need to be performed regarding the Consultation items management screen.

It adds support for managing Consultation items in modal windows taking advantage of asynchronous requests and the already present magnificPopup library. Both creation and edition views can be accessed in modal windows while the old fashioned views are still there as an alternative management workflow.

There's room for improvement here but I'd say it's good to keep iterating quickly and see how things are going.

How should this be manually tested?

Just check that any Consultation item can be added or edited in modal windows as in the screenshot below at http:https://gobierto.dev/admin/budgets/consultations/<consultation_item>/items:

In addition, check that the old fashioned synchronous views are still there (although not looking exactly the same as before to ensure compatibility) in any of the following URLs:

• http:https://gobierto.dev/admin/budgets/consultations/<consultation_id>/items/new
• http:https://gobierto.dev/admin/budgets/consultations/<consultation_id>/items/<consultation_item_id>/edit

Extra 🎱

Now that we've a pretty stable back-end, I've slightly refactored both GobiertoBudgetConsultation's ConsultationForm and ConsultationItemForm classes for consistency and ease of use.