fix(fxa-client): remove lang from fxa-client requests

Fixes #1404.
fix(tos-pp): rely on accept headers instead of naviagor.language for partial requests

Fixes #1412.
Merge pull request #1413 from mozilla/issue-1404-remove-lang

Remove `lang` from fxa-client and partial requests

rebased, re-pushed, checked German - looks great. r+