Assign token when users hit edit page, TrackedPageContentSaveComplete

Assign token to users (anonymous and logged-in) when they hit the edit
page, or start VisualEditor.

This is partly based on 99286bb24e86185fa7a0759588ec09ee74b2cd8c
(previous token-assignment and instrumentation commit) and
0477aa472a6a49f943097d6431ea7c44ecb310ca (fix to $revision), but the
token-assignment part is different.  Unlike before, we are only
assigning a token when it looks like they're about to edit.

See https://meta.wikimedia.org/wiki/Research:Anonymous_editor_acquisition/Signup_CTA_experiment
for more information.

Change-Id: I8d6dfed8faf9fc087bcf7f3d87c60e1d7721824f
(cherry picked from commit 156bd4a1a4d44591bfd4fae31ea529cf08e17671)