Add missing validators.
Fix bindings for the user's certificate.

Replace the numeric Role enum with a string one; remove the role
service.
Remove the search user panel.
Simplify the style and the template for the user dialog.
Remove alert when changing page size in the search table component.

Upgrade Angular to 6.1.10 (i.e. latest): upgrade to the HttpClient.
Upgrade Angular Material to 6.4.7 (i.e. latest): upgrade to mat-*
elements, attributes and directives; fix date pickers; fix missing
md-input-containers elements.
Upgrade RxJS to 6.2.2 (i.e. latest). Remove RxJS from devDependencies.
Remove unused codelyzer dependency.
Remove no longer needed md2 dependency.
Add section for the certificate to the user screen.
Add certificate service to support uploading certificates to the server.

Add code to support adding, deleting and editing users in the front-end.
Rename username property to userName.
Add roles and a role service.