Move Nav and Toolbar files into new Header folder

[lindapaiste]

We want to move these files before @dewanshDT makes any changes so that the git history is preserved.

Changes:

• Move Nav, Toolbar, and their test files into a folder IDE/components/Header
• Update references

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123