Clean up and make chooser-modal.js more generic

[jeromelebleu]

The following changes are made to chooser-modal.js:

• Factorize the fetch function for search and pagination
• Check for the presence of a search form to add event handlers and listen to field changes in a generic way
• Use FormData to retrieve the search filter value instead of accessing the element by its ID
• Use ES6 const and let keywords to declare variables - I guess linters and webpack environment should be shipped here at some time

The custom _ajax_pagination_nav.html template is also replaced by wagtailadmin/shared/pagination_nav.html, and the submit button is removed.

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This contribute to #44, I tried to limit changes in order to make those PRs easier to review.

[gasman]

Generic chooser viewsets are now implemented in Wagtail 4.0 and serve as the basis of the built-in choosers. https://docs.wagtail.org/en/stable/extending/generic_views.html#chooserviewset