Refactor DDL Claim Letter controller

[samcoforma]

Issue Description

Currently the claim letter controller uses conditional logic to determine when to use mock data. Move this into a mock service instead to simplify and standardize controller code.

Possible improvements to consider

• We have discussed passing the list of accepted document types into the service instead of including the logic in the service itself. This would allow our team / the mobile team to have independent lists of accepted document types
• Currently there is conditional code to either use the VBMS client or to return mock data. This conditional logic exists in both of the methods used by the Claim Letters Controller. It would make more sense to create a dummy VBMS client so that the code in both methods doesn't need to take the Rails env and the vsp env into consideration

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Tasks

• [ ] move mock data logic into mock service
• [ ] remove/simplify conditional logic from controller

Acceptance Criteria

• [ ] Tests pass

[jacobworrell]

@samcoforma we still need to do this?