Traffic Portal build system uses local dependencies

[ocket8888]

Because of how the build system recursively copies the traffic_portal/ directory, existing node_modules and bower_components dependencies directories in the locally-copied repo will be pulled into the build environment. This means that conflicting dependency versions can exist - which can cause the build to fail cryptically. Problem is easily fixed by deleting the local resources, but developers might be annoyed by that, so a better solution would be to just either destroy the dependency directories in the build system before running bower install/npm install or just not copying them in the first place.

[zrhoffman]

This was fixed in #4758, which added git clean -fdX (force, recursive, remove only gitignored files) to clean_build.sh.