refactor: rename handler receiver to something more accurate

The following discussion from !765 (merged) should be addressed:

• @jaime started a discussion:

  nit (non-blocking): a * Handlers is probably a leftover from when the middleware functions were moved out of app.go. The change requires more refactoring so let's also follow-up 😄

  func (h *Handlers) HandleArtifactRequest(w http.ResponseWriter, r *http.Request) bool {