Remove ambiguous characters from tokens

These are handled by humans, and occasionally humans use typefaces
that do not distinguish between certain characters very well.

This is the reason that I and Q are excluded from UK vehicle
registration plates.

Remove the characters zero, one, oh, eye and ell from tokens. Make
them all lowercase, because there's no good reason not to.
Merge pull request #264 from alphagov/remove-ambiguous-letters-from-tokens

Remove ambiguous characters from tokens