Add Optimizely hook to page edit button

Optimizely can track this button by ID, class name, or complex selector.
All of these change from time to time, however, so having a dedicated
attribute will be ideal. It is also self-documenting, making it clear to
readers that the attribute cannot change without coordination with
Optimizely test managers.
Merge pull request #2493 from openjck/optimizely-edit-button-hook

Add Optimizely hook to page edit button