Use redirect actions for user-owned resources on `/me`

[myssto]

The Microsoft.AspNetCore.Mvc.ControllerBase comes with a method RedirectToAction(string? actionName, string? controllerName, object? routeValues). Returning the result of this method will redirect the requester to another action using a status 302 Found. Actions on the MeController are essentially wrappers for for accessing user-owned resources by parsing the authorized user id, so this is a much more sensible way compared to what we currently do by re-using the related service method for obtaining the resource.

For example, consider the endpoint GET /me. This endpoint currently parses the requester's authorized user id, and then proceeds to reimplement the functionality of GET /users/{id}.

    public async Task<IActionResult> GetAsync()
    {
        var id = HttpContext.AuthorizedUserIdentity();
        if (!id.HasValue)
        {
            return Unauthorized();
        }

        UserDTO? user = await _userService.GetAsync(id.Value);
        if (user?.OsuId == null)
        {
            return NotFound();
        }

        return Ok(user);
    }

Here is what it could look like using RedirectToAction()

    public IActionResult Get()
    {
        var id = HttpContext.AuthorizedUserIdentity();
        if (!id.HasValue)
        {
            return Unauthorized();
        }

        return RedirectToAction("Get", "Users", new { id });
    }

[myssto]

Soft breaking change, as response codes will be changed.