When set for an export config, if the user doesn't have that attribute, their email will be substituted as a default. If any of the values is not a valid email address, they are filtered out.
Closes#29
* simplify AttributeEndpointsResourceProvider
* improve explanation of properties in AdminUiPage
* add example for mailing lists and map endpoint
* better explain configuration and operation of provider in main readme
* adjust the Keycloak realm
* Add unit test for endpoint class
Configure Keycloak to allow attaching a debugger (localhost:8081).
Add a testing realm with suitable example entries, and explain how to update and export the realm.
Reviewed-on: #27
