Provide read-only listing of enforced configs

Problem/Motivation

As of this writing, the only way to see a list of enforced configuration in the Drupal UI is by installing config_enforce_devel.

It would be useful to have a read-only list of enforced configuration even when in "production mode" with only Config Enforce installed.

Proposed resolution

Note that we might move this to 3.0.x in the interest of stabilizing 2.0.x.

1. Make a read-only version of the "Enforced configs" functionality from Config Enforce Devel
2. Make "Enforced configs" the top-level admin menu item and default local task/tab (currently it's Config Enforce settings)

Remaining tasks

Plan implementation
Write tests
etc.

User interface changes

Config Enforce users will have access to a read-only list of enforced configs.

There will be URL and navigation changes to make Enforced configs the main landing page on the administrative side.

API changes

TBD

Data model changes

TBD, none expected.