Fix cache problem when non admin users access the site.

There is a bug in the cern_toolbar that caches the username and users
visiting see other people's usernames. This causes major confusion. To
reproduce it I needed to create two service accounts that do not have
administrator access.
Steps to reproduce:
 - in a private window login with srvaccount1
 - observe srvaccount1 in the toolbar and close the private window
 - in a new private window login with srvaccount2
 - observe srvaccount1 in the toolbar again

cern_toolbar/cern_toolbar.info.yml

@@ -3,6 +3,7 @@ type: module
 description: "Provides CERN Toolbar. The CERN Toolbar is obligatory for all CERN Drupal websites so make sure that the checkbox on the left is ticked."
 core: 8.x
 package: CERN Basic Elements
+version: 1.0
 dependencies:
   - cookieconsent
   - locale
\ No newline at end of file

cern_toolbar/cern_toolbar.module

@@ -56,6 +56,7 @@ function cern_toolbar_page_top(array &$page_top) {
       '#type' => 'cern_toolbar',
       '#cache' => [
           'tags' => ['cern_toolbar'],
+          'contexts' => ['user'],
       ],
   ];
 }