Change base URI for admin

A number of developers have indicated they already use "/admin" within their
applications, which means the Apigility admin route conflicts out-of-the box.
Additionally, it would be useful to have the API and UI have distinct URIs, in
part so we can also later have a "docs" URI separate from either the admin or
the API.

This patch makes the following changes to the zf-apigility-admin route:

- The route is now "/apigility" to prevent conflicts in existing applications.
- The route is now non-terminating.
- A new child route, "ui", is used to access the admin UI. This will require a
  change in zf-apigility-welcome so that it will link to the correct route.
Merge branch 'feature/base-uri'

Change base URI for admin to "/apigility/ui".