Improve maximum spaces validation

Problem/Motivation

Working on [#] we identified a possible issue:
we want it to be the case that

if an administrator changes the number of spaces on a registration to exceed the maximum_spaces setting, then other custom fields on the registration can still be edited by the registrant without the registrant needing to reduce the spaces.

RegistrationWithinSpaces constraint needs to be checking the original registration. Or perhaps better using Registration::getAdditionalSpacesRequested() proposed in 📌 Explore alternatives to passing existing registration into hasRoom Active .

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes