Give Veterans the ability to download additional claim development letters

[iandonovan]

Problem

This issue is for implementing the Claim Letters changes discussed in this issue and others.

Here is the original Change Request

Veterans are currently not shown the full suite of Claim Development Letters. We are showing them letters with doctype 184, but no other types. We would like to show them the other letter types. Separately, we would like to make sure that the letters are shown with human-readable names.

Solution

The list of letters we show is managed by this list and Flippers

First, we need to enable the Flippers that control the non-184 letters (184 is always rendered). These are:

1. cst_include_ddl_boa_letters
2. cst_include_ddl_5103_letters
3. cst_include_ddl_sqd_letters

and they are managed in the Flipper UI

So, we'll need to enable those flippers on production. These Flippers only regulate this list of letters, they are not used elsewhere in vets-api. Enabling them will not turn on other features.

Second, we need to add code to vets-api to ensure these letters' names are sent to the client as what we'd like them to be. Currently, the web client and mobile app maintain separate code to render the correct names. If we move this upstream into vets-api, we will remove this duplication and ensure both applications show the same information.

Third, @Sparowhawk noted in slack that The only consideration that needs to happen is to make sure the claim letter has the decision letter sent flag flipped to true in the api/upstream. I don't know what this means. Or, rather, I understand what it means but I do not see a way to set a decision letter flag.

Acceptance Criteria

1. All claim development letters are rendered
2. All rendered letters have the correct user-readable names from this table
3. Each claim letter has a decision letter flag set to true(?) so mobile knows to display it(?)

[iandonovan]

Some more info on the naming.

This seems to be the frontend code that performs the doctype-to-display-name conversion

Analogous code to this probably exists in the mobile app, though I am having some problems finding it.

@janechodance and @dysbo, I think this ticket will require that I move that doctype-to-name mapping into the backend and return it as a new field in the json response. Then, the frontend will require a (small) change to use that field instead of the currently-present mapping. The changes won't need to be done simultaneously, but the backend change will need to happen first.