Issue #849 - Fixed: Updated CI to use new extension build process

This MR contains the following changes:

• Upgraded Node.js dependency from 10 to 12 (see related note).
• Made changes related to ensure_dependencies.py being replaced by git submodules:
  • There's no need to create .git/info/exclude file anymore.
  • Update root path in .gitmodules file instead of in dependencies file.
  • Explicitly run git submodule update instead of implicitly called ensure_dependencies.py.
  • Swap out adblockplusui dependency after the fact instead of updating its dependency in dependencies file.
• Made changes related to build.py being replaced by gulp:
  • There's no need to explicitly run npm run dist anymore.
  • Run npx gulp build instead of build.py build.
  • Attach commit hash to build files after the fact, because we can no longer specify an output file name.
• Fixed: Comment referred to wrong GitLab path.

Since the "build" stage is only running for specific branches, I've created a companion feature branch for this issue to run the full pipeline (see feature-849-build).