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 command and verify that the date_joined field has been replaced by first_active and returns a sensible value.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:

• Code Quality
• Pep-8
• Tests (if applicable)
• Success Criteria Met
• 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.

[MaxVanDeursen]

Do we know anything about the performance of this query? It sounds like it could be quite detrimental to execution time if there's many submissions, especially since they have to be sorted. Wouldn't we be better off just making this an additional field of the user, since this date does not change after the initial claim?

[TimJentzsch]

I think it would be quite fast, but it's probably better to do it as @itsthejoker suggested and to just fix the date_joined field to actually be correct

[TimJentzsch]

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