[recnet-api] Slack oauth API

[joannechen1223]

Description

This PR is the backend part of migrating slack app to the oath access token, which will resolve the limitation of single workspace.

Doc: https://www.notion.so/Slack-OAuth-flow-14551bd6607c80d08a13f56c04a81abd

What's included in this PR:

1. Add following API endpoints:

• GET /users/subscriptions/slack/oauth: ger user's slack workspace name they installed
• POST /users/subscriptions/slack/oauth: Post code from FE to slack API to exchange the access token, store information in DB
• DELETE /users/subscriptions/slack/oauth: delete the user's slack information

2. Add back slack test API POST /subscriptions/slack/test
3. modify SlackTransporter logic to use the user's accessToken when sending messages
4. Add new slack fields and delete slackEmail fields in user table

Related Issue

Notes

1. AccessToken is encrypted in the storage and is decrypted before it needs to be used. Encryption key is stored as an environment variable.
2. Backward compatible. If the user has subscribed Slack channel without a user access token, we will use the one in env var.

Test

1. Run local server
2. Pasting this URL in your browser (fill in the scope and client_id)
   https://slack.com/oauth/v2/authorize?scope={scope}&client_id={client_id}

3. Choose a workspace and click allow button, pull the code in the callback request from devtools

4. Use the code to hit `POST /users/subscriptions/slack/oauth` 5. Hit `POST /subscriptions/slack/test` to see if you got the slack message 6. Hit `GET /users/subscriptions/slack/oauth` and `DELETE /users/subscriptions/slack/oauth` to see if it works correctly

Screenshots (if appropriate):

N/A

TODO

• Clear console.log or console.error for debug usage
• Update the documentation recnet-docs if needed
• Update environment variables in staging and production env
• Add unit tests
• Add migration announcement with specific deadline

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2 Skipped Deployments

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[swh00tw]

I think we should add some document somewhere (maybe in notion or use comment or both) about

1. How you do the encryption/decryption (which algo we choose)
2. The decision and more context about backward compatibility

[swh00tw]

Great works! I've tested the endpoints. Please fix the failed test before merge.