Add more headers to Access-Control-Allow-Headers

We recently added functionality to proxy request headers to backend
services.

See https://github.com/alphagov/spotlight/pull/813

This code also runs in the browser, so we need to ensure that it isn’t
restricted by browser security. The other option would be not to try to
set those headers in the client.
Merge pull request #364 from alphagov/bugfix/spotlight-cors-request-headers

Add more headers to Access-Control-Allow-Headers
Just use autopep8 as part of the build

If a computer is going to moan at me about whitespace, a computer can
fix it.
Merge pull request #365 from alphagov/feature/autopep

Just use autopep8 as part of the build
DRY Request-Id processing
Merge pull request #366 from alphagov/feature/dry-request-id

DRY Request-Id processing
PEP8
Merge pull request #367 from alphagov/flaskutils-pep8

PEP8