Remove toolbar code from gitlab-ce and add back built script

After a chat with @mikegreiling, it seems like a good way to integrate the script into gitlab-ce while building it in this separate project will be to:

• release the script as an NPM package
• add the package as a dependency in gitlab-ce
• use something like CopyWebpackPlugin to take the file from node_modules and place it at its current path in assets/webpack

This must be done after #2 (closed). But should be done soon, so we don't have to balance code and tests in two spots.

We can then add documentation about accessing it via Unpkg as discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/62541#note_191266732