[CST] Website, Mobile, and Virtual Agent should retrieve the same claims letters

[iandonovan]

While working on https://github.com/department-of-veterans-affairs/va.gov-team/issues/96224 I discovered that

1. We have feature flippers in place to regulate the types of Claims Letters we retrieve and show veterans
2. Only vets-website uses those feature flippers. Mobile and Virtual Agent are both configured to only retrieve doctype 184. (Mobile additionally has a secondary gate to exclude doctype 27 even if the flipper enabling doctype 27 is turned on elsewhere.)
3. No one can think of a reason for this to continue being true.

So, in an effort to both clean up code and make the user experience more consistent, I should shift the logic around using these flippers into the Downloader service itself, rather than the three individual controllers. This means the three places will all show the same letters.

Here is a link to my discussion with the Virtual Agent team.

Here is a link to my discussion with the product team.

Here is a link to my discussion with the mobile team.

[dysbo]

I think this PR is related to this? Adding comment to link the two for convenience