Enhance feature specs around registration for readability

What does this MR do and why?

• moves registration via invite into its own spec file
  • Did this to help self document/achieve goals laid out in #373194 (closed)
• creates a shared setup concept to be used through out #373194 (closed) with saas_registration
• attempt to make a closer-to-human readable example spec
• combined 2 examples into 1 and went from start to finish - getting closer to e2e
• modifies method names and adds comments to help lead to ease of readability and discovery

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #373194 (closed)