GAE setup should set entire source repo tree with `pixl` group ownership

[jeremyestein]

Definition of Done / Acceptance Criteria

• Set entire repo (eg. /gae/pixl_dev to be owned by group pixl rather than the mixture of docker and pixl as at present
• Change docs in docs/setup/uclh-infrastructure-setup.md

Testing

No response

Documentation

No response

Dependencies

• pixl user and group will need to be created on GAE
• pixl dev users will need to be added to pixl group

Details and Comments

Because of #400, data that a PIXL container needs to be able to read must have group ownership of pixl, not docker. That is, the exports directory. Why not simplify it and make the entire repo owned by pixl?