Add ability to run a 50% split user test with VisualEditor as default

Will only run if $wgVisualEditorEnableSplitTest is set to true. To be removed
once VisualEditor is fully deployed.

Bug: 49604
Change-Id: I5c603ece309d61641d32ccc9eff5ea2890d5b816
Merge "Add ability to run a 50% split user test with VisualEditor as default"