[feature idea] Add skip_group and skip_form prompt types

[nathanathan]

There are some cases that are difficult to implement with relevance groups. For example:
https://groups.google.com/forum/?fromgroups#!topic/opendatakit/NUA9weJAgUM

I think we could make these cases a bit simpler by adding a few "macro" prompt types (i.e. prompt types don't translate directly to xform elements but instead generate a bunch of xform logic to do something).

skip_group would "and" its negated relevance condition onto to all the prompts after it within same group.
skip_form would "and" its negated relevance condition onto all the prompts after it.