Caching for get_registration_purposes endpoint

[BCerki]

###Description of the Tech Debt###

Add caching for get_registration_purposes endpoint (We already have the cache invalidation in the data model)

###Tech Debt Triage#
The purpose of our technical debt triage process is to analyze technical debt to determine risk level of the technical debt and the value in tackling that technical debt.

Risk Value Scoring:

Level	Value
High	3
Medium	2
Low	1

Technical Debt - Risk Types	Level	Value
Business Area Risk - Risk of business area visibility / damage to user experience
Developer Fault Risk - How likely will this tech debt cause a future error related to coding on top of it
System Fault Risk - Risk of system errors or application downtime
Time Scale Risk - Compound risk effect if left alone. How much more difficult to fix or dangerous will this become over time?
Time Sink Risk - How much will this tech debt slow the development process down
TOTAL SCORE: