Support Setting A Model's Search Field Through A Setting

[dchukhin]

Use Case:

The use case for this feature is a project using Django's default user model, with a Page that has a relation to the user model. Since Django's default user model does not have a title field, it's not possible to use wagtail-autocomplete on the relation. One option would be to define a custom user model and a title field, but doing so mid-project a difficult task. Instead, it would be great to be able to point the search to a different field on the model without having to edit the model.

Changes

This pull request:

• updates the views.render_page() function to check (only if the page doesn't have an autocomplete_label attribute) for a WAGTAILAUTOCOMPLETE_CUSTOM_SEARCH_FIELD setting, and (if the setting is defined) to use the value of that setting.
• updates the views.filter_queryset() function to check (only if the page doesn't have an autocomplete_search_field attribute) for a WAGTAILAUTOCOMPLETE_CUSTOM_FILTER_FIELDS setting, and (if the setting is defined) to use the value of that setting.
• updates the documentation based on these changes

As a result, it is now possible to define which field(s) to use in the search without having to edit the model.