Backend: use consistent api route conventions

[dominiquegarmier]

I would suggest something along the following lines:

• change all base endpoints to plural, i.e.: file -> files, mission -> missions, etc
• all GET endpoints that always return either a 404 or a single item should not use query params and should use a url (:uuid, etc) param. I.e. we should make more extensive use of :param fragements and REST verbs.
• ex: GET missions/many should become GET /missions
• ex: POST missions/create endpoints should just become POST /missions
• we need to think about how we want to "query one by name" either we use the paginated endpoints and request a single item. Or we have endpoints like projects/:porjectId/missions/:missionId and then projectId and missionId can be names or uuids. (personally I think going the paginated route is much easier tho makes the typing slightly less strict.)

[dominiquegarmier]

once i extend the cli funcitonality I can create e2e tests and pretty confidently rename these things one by one.

for now it is better not to break stuff and be careful.

[wp99cp]

resolved with #2289