Provide a helper for linking to another app's route by role #317

Description

Apps that link to each other's pages currently call getUrlByRouteRole and then hand-roll the same logic: substitute the :courseId param, test whether the result is an internal path or an external URL (a scheme regex), and pick between a react-router Link and a plain anchor with a legacy fallback URL.

This exact pattern is now duplicated verbatim in two apps: frontend-app-gradebook's "Back to Dashboard" link (openedx/frontend-app-gradebook#627) and frontend-app-instructor-dashboard's "View Gradebook" button (openedx/frontend-app-instructor-dashboard#245). Any app that cross-links by role will need the same boilerplate, and the copies will drift.

frontend-base should own this. Options include an isAbsoluteUrl helper plus param substitution, a getUrlByRouteRole variant that also reports whether the result is internal or external, or a RoleLink component that takes a role, params, and a fallback URL and renders the right element. Once available, both apps above should be migrated to it.

Provided by #320, released in 2.0.0-alpha.15, and adopted in:

• refactor: resolve the instructor dashboard link with frontend-base frontend-app-gradebook#631
• refactor: drop the local route-by-role helper for frontend-base's frontend-app-instructor-dashboard#248
• refactor: resolve course-about links by role with frontend-base frontend-app-catalog#165
• refactor: resolve the account settings link with frontend-base frontend-app-notifications#135
• fix: resolve the dashboard route with frontend-base frontend-app-authn#1693
• feat: link Discover to the catalog's courses route when the site has one frontend-app-learner-dashboard#873