Add cache usernames table and integration test playbook

Changes:

• list of Slack Oauth Scope permissions granted to the bot in the README
• add a DynamoDB table for caching usernames. you can choose to disable this feature by commenting out the CACHE_USERS_TABLE environment variable in serverless.yml and the DynamoDB file in serverless.yml
• add update_cached_users lambda function for automatically populating the username table
• add an "integration test" playbook that chains together all the lambdas intended for use in playbooks (not the endpoints).
• add an integration test "test input" that is uploaded to SSM. This file's contents can be used to trigger the slack integration test playbook via the create_events function in the main socless stack. NOTE: you will need to modify this file's event details for your Slack environment
• reduce duplicate code among all functions by updating the ./common_files/slack_helpers.py file with generic functions and improved error handlng

Initial release to support with CI/CD deployments

This is an initial release to support with CI/CD deployments