Use Google cloud's Application Default Credentials

[MrOrz]

• refactor: Use google's application default credentials instead of using DAILOGFLOW_CLIENT_EMAIL
  • In theory, "service accounts" should represent a "service" accessing the Google Cloud API, and we configure the what this service can access on Google Cloud.
  • In this case, the "service" should be Cofacts LINE bot as a whole, not "The part of LINE bot that accesses Dialogflow". On Google Cloud, we configure the service account representing Cofacts LINE bot to have access to DialogFlow, and other APIs in the future (such as BigQuery).
  • After this PR, we can reuse the same service accounts with BigQuery.
• refactor: remove unused google drive script
• fix(babel.config.js): let babel module-resolver recognize ts file
  • Without this fix, import to ts file in in webhook/handlers/initState will not be transformed into relative path by module-resolver, causing "Cannot find module 'src/lib/detectDialogflowIntent'"
    runtime error

When deploying this, we should do the following:

1. Create a new service account (and its credential) representing Cofacts LINE bot (both staging and production) as a whole
   • Ensure it has permission to access Dialogflow (see README for the required permission)
2. Setup GOOGLE_APPLICATION_CREDENTIALS for rumors-line-bot (Changes to rumors-deploy: Update line bot service account's keys rumors-deploy#23)
3. Update rumors-line-bot image and test if Dialogflow integration still work
4. Cleanup old environment variables

After we repeat the above process for both staging and production environment, we can cleanup old service account that is specific to Dialogflow only.

[github-actions]

Pull Request Test Coverage Report for Build 4822667690

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 91.558%

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Totals
Change from base Build 4822470920:	-0.02%
Covered Lines:	962
Relevant Lines:	1030

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💛 - Coveralls