Browser extension not working on Phabricator

[felixfbecker]

Go to https://phabricator.sgdev.org/D1
There is no code intel, no view file buttons, no extension debug button, nothing.

Unclear whether native integration is affected too.

[felixfbecker]

This seems to be because the newInject feature flag is off by default for Phab, and doesn't work with 3.x instances.
#2909 fixes this.