Story 1620 (part 1): Update leftNav Signup A/B test for new schema

The page-edit-impression action has been divided into 2 separate
actions: page-edit-click (with clickSource) and page-edit-impression.
This is mostly as a QA assurance.

Will move A/B test to stable in a follow-up commit.

Change-Id: I825158438a88de9571f7775e837b550475de2dab
Merge "Story 1620 (part 1): Update leftNav Signup A/B test for new schema"