Remove several registration fields that are not required anymore

[glbert-does]

when ready, this will close #488

• rating (is fetched from the site via the api)
• peak rating (could be fetched from the site and maybe still is; but this has not been used in ages anyway)
• previous season alt (we know who was an alt last season – i am not completely sure everything works correctly here though)
• already in slack group (we know that, it's players who are linked)
• slack_username (as far as i know, this hasn't been used in years)

i removed "expected rating" at the same time. this hasn't been used in ages, and cannot even be used with the current team gen algorithm. frankly, i am not certain it ever made sense to use it.

edit: now also closes #489

[lakinwecker]

I haven't finished a full review, but I thought I would send my first comments anyways. I'll want to do a deeper review and test this a bit, which don't have time for today. But first let's deal with changing the old migrations. What was the intent for those?

[lakinwecker]

Altering old migrations has no effect on the live database, what was the intent with this change?

[glbert-does]

the intent was simply to make the new migrations revertible. as far as i can tell, that's the only thing it does, and i didn't see a downside to it. i don't mind removing the changes to the old migrations from the pull request.

[lakinwecker]

I am pretty sure there are ways to make the new migrations invertible that don't include modifying old migrations. The problem with modifying old migrations is that our live database will not be in sync with this change, which means that new migrations may assume they can do things that they can't and it will succeed in development, but not on prod. Let's remove the changes to old migrations and if you want the new migrations to be revertible, we can work through that.

[glbert-does]

now also closes #489