teams: implement team roster SD-269

Summary:

• Start using dedicated files for storing models (enums, DTOs, params, util functions, etc): we really need a better organisation, as scrolling 1000 lines is not efficient. We should mirror the same organisation as in BE ideally.
• Use the exact same name as in BE: It's a nightmare to be sure that we are using the same DTOs when they are not called the same. Looking for one day having shared folder for them, we really need to use the same names.
• Create DTOs and specialised slices according to the need: objects are correctly typed, and no more fetching a long list when we are only looking for a single item (GET /teams/:teamID/roster or GET /accounts/:accountUUID/rosters vs GET /accounts/:accountUUID/rosters/:teamID)

Closes SD-269

FE of hq-tech/backend!593 (merged)