Fixed member joinedAt being overwritten for the Slack and Twitter integrations

[joanreyero]

Changes proposed ✍️

When we onboarded we set the follow timestamp to 1970, and the member’s joinedAt as well. So far so good.
But when we onboard, we upsert that same activity with timestamp now(). The upsert for activities was working fine. However, when it was calling the upsert for members, it was sending the timestamp now() as joinedAt , when it should have been sending the timestamp of the existing activity.

Checklist ✅

• Label appropriately with Feature, Enhancement, or Bug.
• Tests are passing.
• New backend functionality has been unit-tested.
• Environment variables have been updated:
  • Local frontend configuration: frontend/.env.dist.local, frontend/.env.dist.composed.
  • Local backend: backend/.env.dist.local, backend/.env.dist.composed.
  • Configuration docs have been updated.
  • Team members only: update environment variables in override, staging and production env. files and trigger update config script.
• API documentation has been updated (if necessary) (see docs on API documentation).
• Quality standards are met.
• All changes have been tested in a staging site.
• All changes are working locally running crowd.dev's Docker local environment.