[SPARK-11344] Made ApplicationDescription and DriverDescription case classes

DriverDescription refactored to case class because it included no mutable fields.

ApplicationDescription had one mutable field, which was appUiUrl. This field was set by the driver to point to the driver web UI. Master was modifying this field when the application was removed to redirect requests to history server. This was wrong because objects which are sent over the wire should be immutable. Now appUiUrl is immutable in ApplicationDescription and always points to the driver UI even if it is already shutdown. The UI url which master exposes to the user and modifies dynamically is now included into ApplicationInfo - a data object which describes the application state internally in master. That URL in ApplicationInfo is initialised with the value from ApplicationDescription.

ApplicationDescription also included value user, which is now a part of case class fields.

Author: Jacek Lewandowski <lewandowski.jacek@gmail.com>

Closes #9299 from jacek-lewandowski/SPARK-11344.