Move from RootAction to View Action

[janfaracik]

This PR changes the Purge build queue from being a root action to being a view action instead. Root actions have been moved to the header as of jenkinsci/jenkins#10245, and in that move the ability to have custom action.jelly files has been removed - this is in an effort to enforce consistency between root actions.

As a result, this plugin got broken. This PR aims to resolve that by moving the action from a RootAction to a plain ol' Action - so it'll still appear on views as expected, but it won't appear in the header.

Requires jenkinsci/jenkins#10676 to show the view action with the new header.

Testing done

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