Handle select box with long list of options

[fpietrosanti]

This ticket is to analyze the UX issues and propose a solution to handle select box with very long list of items.

For example, the list of all the italian public administration (40.000 entries) must be tested in the current submission field design.

[NSkelsey]

A great solution is to use a searchable select box. https://harvesthq.github.io/chosen/

How to integrate search into the backend is another question entirely.

[NSkelsey]

In the interest of incrementalism, the goal is to introduce this change as an option in the UI for receivers and context selection first. If we find it useful we can port it into a field template.

The goal here is to add support for the selection of tens to hundreds of contexts and receivers while making the selection easy™ @evilaliv3 has suggested we use http://angular-ui.github.io/ui-select/

[NSkelsey]

Current UI with context.description and 200 Italian public administrations loaded and sorted via the group by selector.

[NSkelsey]

The UI becomes quasi unusable as more elements are added. It is almost unusable at 500 contexts. The search functionality responds very slowly.

There is a tradeoff to make on look and feel and responsiveness. The current selector committed seems to be fine with ~200 elements. We will adjust accordingly if larger numbers are needed. The first thing to improve efficiency is to remove the text highlight in results.

[NSkelsey]

Further the submission UI needs some hint that have selected the right context by displaying some of that information on the subsequent whistleblower web panes.