Add first_active field to BlossomUser model

[TimJentzsch]

Relevant issue: Closes #141.

Description:

The Slack info command used the date_joined field of BlossomUser. However, this is actually never assigned anywhere, so it always returned the current time. This adds a separate first_active field which returns the time of the first claimed transcription. This is then used instead of the date_joined field for the info command.

Testing Instructions:

Locally setup the Blossom Testing App. Then you can test the info <username> command and verify that the date_joined field has been replaced by first_active and returns a sensible value.

Because I didn't manage to set up Slack locally yet, it would be nice if someone could verify this for me.

Checklist:

• [x] Code Quality
• [x] Pep-8
• [x] Tests (if applicable)
• [x] Success Criteria Met
• [x] Inline Documentation
• [ ] Wiki Documentation (if applicable)

[itsthejoker]

date_joined is actually when the object was created in the database, so anyone who accepts the code of conduct now has an accurate date_joined field. What would probably be a better option here is going through and resetting that field in the db to be the claim time of their first submission if we have that available.

[TimJentzsch]

I will close this and instead we can fix the date_joined field.