Nxcm 4665 scheduled task to remove releases

Added a new scheduled task that closely follows existing logic in SnapshotRemoval task.
Added a new UI component to display a combobox for selecting Repository Target to apply(optional).
New task will walk a repository and delete older RELEASED versions if more than the specified number to keep exist. A Repository Target can be applied to narrow the traversal.

https://issues.sonatype.org/browse/NXCM-4665