Ignore needs to become a separate state

[Bios-Marcel]

Ignore is currently a magic value expected in tags. It's hard to remember and its existence is invisible to the user.

Instead there should be a boolean field called ignore with default value false.

This state should be settable via classifications in addition to the tag, but also via the "detail section" in the payments table.

A payment can both be classified with a tag and ignored, even if the tag won't be used in evaluations.