build: fetch GQL schema from endpoint

[felixfbecker]

This uses the same mechanism as in the webapp to fetch schema.

[felixfbecker]

So, there are now some compile errors - in theory they should go away if ecc gets updated to the latest schema.

[chrismwendt]

Here's how to move this PR to https://github.com/sourcegraph/sourcegraph/tree/master/packages/browser-extensions

cd browser-extensions
git format-patch master --stdout > /tmp/patch

cd ../sourcegraph
cat /tmp/patch | git am -3 --directory=packages/browser-extensions/
# and fixup merge conflicts