support deleting all assets under entire selected tree nodes

In Nexus 2.x you could right click a Browse Storage tree node and choose to delete it. This is extremely handy and also is something we rely on instructing end users to do to solve problems with Nexus 2.x

Nexus 3.x tree view should have a similar feature. When the node is deleted, appropriate background threads update whatever appropriate metadata is associated with it in response.

Having this feature at least for maven 2 hierarchies would maintain functionality customers expect.

Acceptance

• There is a way for a user to delete a node.
  • A panel? A button at the top? A panel may be useful for showing other information later, e.g. file count, total size, etc. A button right in the row?
• That option should be enabled if the user has permission to delete at least one asset
• The delete operation should be confirm (since they may delete a lot of stuff)
• The operation should delete whatever the user has permissions to delete.

Notes

• It may be necessary for the deletion operation to be asynchronous, since it's so much content.
• Are there any affinities with the existing staging code that deletes and/or moves search results? (Those are synchronous, however.)

Note the spike for impl plan has been completed see this comment for details https://issues.sonatype.org/browse/NEXUS-17880?focusedCommentId=522630&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-522630