Allow search custom fields on a custom user model

Is your proposal related to a problem?

Current wagtail admin user search only allows search on four hard-coded fields: username, first_name, last_name, and email. Would be good if developers can specify custom fields via User.search_fields.

Describe the solution you’d like

If User.search_fields exists, join extra fields to the search term.

    if 'q' in request.GET:
        form = SearchForm(request.GET, placeholder=_("Search users"))
        if form.is_valid():
            q = form.cleaned_data['q']
            is_searching = True
            conditions = Q()

            for term in q.split():
                if 'username' in model_fields:
                    conditions |= Q(username__icontains=term)

                if 'first_name' in model_fields:
                    conditions |= Q(first_name__icontains=term)

                if 'last_name' in model_fields:
                    conditions |= Q(last_name__icontains=term)

                if 'email' in model_fields:
                    conditions |= Q(email__icontains=term)

===>      # TODO:  check search_fields  and join it.

            users = User.objects.filter(group_filter & conditions)

Describe alternatives you’ve considered

n/a

Additional context

One issue is that the matched content is not visible via the user listing, but developers can just override the list.html to show custom fields.

[y-martinez]

@favoyang I already push the PR 😄 thanks for your help, I am waiting for a review

[favoyang]

@imansyed000 LGTB, wait for a core member to review.