Refactor users service from go to python

[Adibuer-lab]

Issue #, if available:
#13
Description of changes:
Refactor users service from go to python

1. New users service created under users folder with old go based service moved to users-old for reference during PR
2. Changed from in-memory storage of users to using dynamodb
3. Updated cloudformation templates to deploy users dynamodb table with appropriate permissions for users service to access table
4. Added custom resource and lambda function to load users table with shoppers during deployment of retail demo store
5. append unique string to cloudfront request policies

Description of testing performed to validate your changes (required if pull request includes CloudFormation or source code changes):
Integration testing using testing scripts and tested by deploying into aws account

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[Adibuer-lab]

@gfaires ready for review again. Sorry for wait, wanted to make the auto-select shopper and select shopper based on persona and age faster, but pynamodb didn't support sort key queries on GSI's. Ended up reverting to flask, and base dynamodb api's. but now things are much faster, for example testing script which used to run through all endpoints in 3 seconds does it in 0.8 now