cmd/pomerium : refactor main to more testable (#112)

- cmd/pomerium: refactor global timeouts to be configurable
- cmd/pomerium: add tests
- cmd/pomerium: remove debug flag, set with env vars only
- cmd/pomerium: global ping now returns version not OK
- proxy: validate shared secret encoding and length
- docs: add timeout to example policy
- docs: document timeouts and cors
- docs: update pomerium logo
- docs: add policy authorization docs

docs/.vuepress/public/logo-no-text.svg

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