Registered fieldsets with registered fields were always being shown,

even if none of the fields inside were visible. This was because we only

determined visibility based on registered classes, and didn't even

instantiate fields until we had begun rendering the fieldset template.
This was prominent when installing Power Pack 5, which registers a

fieldset for SOS. This is shown but empty on any review request not

backed by SOS.
This change moves field construction out of the template rendering and

into the fieldset itself. A new build_fields() method is responsible

for instantiating all the fields, and a fields property returns a

cached view of that. This is combined with a new should_render

property, which considers visibility of the contained fields.
That simplifies templates and fixes visibility, but it also opens the

door to dynamic fields/fieldsets. A long-term project we've had is a

field editor for Power Pack, and that requires a way to dynamically

create new fieldsets and fields without first registering them globally.

This change enables the dynamic fields part, leaving only dynamic

fieldset generation remaining.
There are also some documentation fixes and a few fixes for state

assumptions noticed during testing.

Verified that the SOS fieldset wasn't being shown outside of SOS-based

review requests.
Added new unit tests, which all pass.
Tested all the current fields we show on review requests.