crmbWizardCtrl.isSelectable() - Don't allow reentry to invalidated step
crmbWizardCtrl.isSelectable() - Don't allow reentry to invalidated step

Generally, the flow of the wizard is that you must complete the preceding
steps before advancing to the next step.

Before this commit, you could trick it into proceeding past an incomplete/invalid
step by:

 1. Fill in step A
 2. Continue to step B
 3. Jump back to step A
 4. Delete data from step A, making it invalid
 5. In the top navbar, jump forward to step B

This resolves the issue.

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