Problem/Motivation
When installing with the standard profile the front page is set to node. On all other profiles (including testing and minimal), the front page is set to user (#375397-91: Make Node module optional) and recently to user/login. However, the login page behaves different for authenticated users and that leads to tests being forced to implement workarounds (e.g. \Drupal\system\Tests\System\AdminTest::testCompactMode()).
Also when installing with the minimal profile, the frontpage shows the login-form as well as the login-block in the sidebar.
In addition, this will help with test-failures over in #2288911: Use route name instead of system path in user maintenance mode subscriber.
Proposed resolution
Introduce a default system frontpage (on system/frontpage) showing the simple message No front page has been configured yet.
Remaining tasks
User interface changes
Front page is set to system/frontpage unless changed by the install profile.