Database structure changes

[abhshkdz]

• Rename uid to user_id, pid to problem_id, time_created to created_at, time_updated to updated_at
• Change score and state to more descriptive names. It is unclear if they refer to difficulty score or similarity score or something else.
• It might be a better idea to change is_deleted to deleted_at and save the UNIX timestamp when the row is deleted, NULL otherwise.
• For all problem IDs, store just the numeric ID and not erd27 and so on. Create a separate table called platforms and a corresponding column called platform_id in recommendation tables if you want to keep it generic and not Erdős-specific. Same for Erdős handles: use username directly and not erdabhshkdz and so on.