Require ENABLE_PONTOON=1 during frontend build to include Pontoon JS in pages

[lmorchard]

Fixes #1356

[lmorchard]

This should exclude the Pontoon script tag by default on all pages, unless ENABLE_PONTOON=1 is set during frontend build. That's enabled in this PR for develpoment-static branch, but it should be off for any other deployments in the future.

You can test this locally by running gulp and verifying that there's no Pontoon script tag on the page. Restart with ENABLE_PONTOON=1 gulp and the script tag should appear.

[chuckharmston]

This looks great!

Only nit: this seems like the sort of thing we should be documenting.

[ghost]

Thanks for cooking this up, Les and for reviewing it, Chuck

[lmorchard]

Only nit: this seems like the sort of thing we should be documenting.

Definitely a good point. One caveat is that the static build system isn't documented at all yet, so there's not really a good place to put just the mention of this flag right now. Hoping to work on those docs today for #1350

[lmorchard]

May collide with PR #1366, but I added a section to deployment docs describing how ENABLE_PONTOON is used.