Implicit discardable tags

[Zaczero]

When processing new changes, we want to automatically discard certain tags so clients don't have too. JOSM and iD already implement such features but we should apply it globally for simplicity.

The goal is to:

• efficiently discard certain tags during tagsvalidating validation function (it should return modified dict of tags), see models/validating/tags
• migrate rules from JOSM and iD

I think if the discard method is simple enough (which is very likely), it should be a part of the validating model file.