fix(server): Add maxAge cache control for static assets

- add configuration option for max age of static assets
- update static middleware to use maxAge value

ref #656
fix(server): add cache-control header for /config
chore(config): disable cache locally
Merge pull request #873 from mozilla/issue656-static-asset-caching

Merge in fixes for cache control
refactor(client): Separate `/force_auth` specialized code from `/sign_in` code.

first step in #762