Switch database migration engine

[laggron42]

Due to the numerous problems we've had with aerich and its limitations, I decided to ditch it in favor of Django's own. However, changing migration engine mid-project for something open source is very perilous.

The following situations must be thouroughly tested:

• Creating a brand new instance
• Updating a bot with up-to-date schemas (all existing aerich migrations were run)
• Updating a bot with out-of-date schemas (updating several versions at once and missing migrations)
• Generating new migrations afterwards

This PR must be merged before creating any new database migration.