Neatening up GCP credentials setup

[apricot13]

This PR makes the setup for GCP buckets much neater and less likely to cause errors on your local environment if you don't have everything setup yet. (I was getting fed up of that header error.) Should help fix the error on #256

one step closer to secrets management!

I have tested locally and on tpx demo staging https://tpx-outpost-demo-staging.herokuapp.com/

Deployment notes

To use GCP locally on development

• download the keyfile.json for the bucket you want to use (1password) and drop it into your /config/secrets/ folder (rename it gcp.json)
• Set your local vars - you wont need any other GCP_* ones anymore

GCP_PROJECT=bucket_name
GCP_BUCKET=project_name

• when running locally it will use this bucket for storage

To use GCP on heroku

• As above download the keyfile.json for the bucket you want to use (1password)
• update the lines below to match your json file - along with the bucket and project names

heroku config:set -a heroku-app-name \
GCP_BUCKET=bucket_name \
GCP_PROJECT=project_name \ 
GCP_APPLICATION_CREDENTIALS="$(< config/secrets/correct-keyfile-for-env.json)"