* Set up shared EnterpriseSearchPageTemplate component

* Set up product-specific page templates + setPageChrome

+ misc tech debt - create AS components/layout/index.ts for imports

* Set up navigation helpers for EuiSideNav usage

- Update react_router_helpers to pass back props as a plain JS obj instead of only working with React components (+ update react components to use new simpler helper)

- Convert SideNavLink active logic to a plain JS helper

* Set up top-level product navigations

NYI: sub navigations (future separate PRs)

* Set up test_helpers for inspecting pageHeaders

- primarily useful for rightSideItems, which often contain conditional logic

* Initial example: Convert RoleMappings views to new page template

Minor refactors:
+ remove unnecessary type union
+ fix un-i18n'ed product names
+ add full stop to documentation sentence
+ add semantic HTML tags around various page landmarks (header, section)

* EUI feedback: add empty root parent section

* Revert Role Mappings union type removal

- but shenanigans it a bit to take our i18n'd shared product names (requires as const assertion)

- done to reduce merge conflicts for Scotty / make his life (hopefully) a bit easier between ent-search and Kibana

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>