Finetune severities

[maschino]

Currently most (if not all) rules are set to severity error. I think we should fine tune them and push some of them back to warning only. I'd suggest the following changes:

• informative-docs
  I'm not sure either when this one came around - it nags about the jsdocs being useless.
• ordered-imports
• semicolon
• no-unnecessary-semicolons
• no-any
  This is a tough one. But I'd say keep it as a warning for now. It would make porting stuff to typescript easier.

I'd like to make tslint part of our internal build pipeline, but I think that the current rules are a bit too strict for that.

What do you think @christophka, @huidini and others?