Introduce preference filters

Introduce preference filters

This normalizes handling of transformations on the boundaries between
preferences and generic form controls and removes the special case
where email-blacklist is passed around as an array internally, leaking
into the API.

As a result of this normalization, meta=userinfo no longer returns an
array of users, using the internal representation like action=options.

Bug: T198935
Change-Id: Iff63da0d215585cfcf083e7f7ec8ed45d5b77301