[cleanup] improved properties file

the current properties file misses some documentation
and also contains some legacy, unused properties.
this commit tries to clean this stuff up:
  * adds documentation for org.osiam.mail.*.linkprefix, and
    org.osiam.html.*.url
  * sets a sensible default value for the above mentioned
    properties
  * removes unused property org.osiam.html.register.url, and
    org.osiam.html.register.url
addon-self-administration