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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[jtnord]

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.

is a bug not a feature JENKINS-75683 which is addressed in jenkinsci/jenkins#10678 (which reveals an existing bug in this plugin -> #126 )

[jtnord]

Obsolete now that #126 is merged?
Additionally actions that affect Jenkins globally rather then just the current context should IMO always be RootActions and appear in the header. Only things that perform something in the context of the current ModelObject should be a view action.