Transifex is sunsetting anything below v3 later this year. Let's
get ahead of our scripts unexpectedly failing.
- Switch 'reactifex' dev dep to our maintained fork at
'@edx/reactifex' (it has an older released version number,
but it's still maintained unlike the original)
- Update Makefile commands for new v3 transifex API
Note: I tried directly invoking `codecov`, but it "couldn't tell" that
it was running in a GA/CI container, so I've opted for using the GA
codecov integration, which does work as expected.
Specifically, make sure that the header, footer, and tabs are all
shared code so that they look the same and don't need to be
redefined as we add more tab pages.
