Feature: Customizable Base Rules

[patrickcuagan]

This MR does the following:

• Creates a missing merge migration so projects that uses this PR will have its CI pass.
• Add an option to exclude default segments (or rules). Not every project will use all available rules so we make it optional.
• Updated the classname of the segment links added to the Wagtail userbar.
• Update help text of a few fields to be more helpful to editors.
• Sort list of countries in origin country rule by country name for better editor experience.

[thibaudcolas]

wuuuhuuu!