Fix missing transcription data

[TimJentzsch]

This includes:

• The create_time, claim_time and complete_time field for submissions
• The date_joined field for volunteers
• The create_time field for transcriptions
• Any dummy transcription data that's still in the system

⚠️ This issue is important, because it results in almost all stats from Buttercup being wrong. ⚠️

[TimJentzsch]

Ideally, we would do #149 first and then already add the title when going through the posts.

[TimJentzsch]

This is done and dusted!