Use session for storing fieldset for next request

[Casperhr]

The pattern you are showing right now makes the browser remember the request as submit, and thereby refresh will submit again.

I created a flash system which (Will create a package just for that soon) (https://github.com/nodes-vapor/admin-panel/tree/master/Sources/AdminPanel/Support/Flash)

But I was doing the same pattern for this

1. Middleware to add fieldset to session from response storage
   https://github.com/nodes-vapor/admin-panel/blob/master/Sources/AdminPanel/Support/Middlewares/FieldsetMiddleware.swift

2. Extension to response for easier adding fieldset
   https://github.com/nodes-vapor/admin-panel/blob/master/Sources/AdminPanel/Support/Extensions/Response%2BFieldset.swift

3. Pull out fieldset from Form, either from session or the static let
   https://github.com/nodes-vapor/admin-panel/blob/master/Sources/AdminPanel/Support/Extensions/Form%2BSession.swift

This will make it possible to make 2 funcs, 1-view 2-submit

View

public func create(request: Request) throws -> ResponseRepresentable {
        return try drop.view.make("BackendUsers/edit", [
            "fieldset": BackendUserForm.getFieldset(request)
        ], for: request)
    }

Submit

 public func store(request: Request) throws -> ResponseRepresentable {
        do {
            // Validate
            let backendUserForm = try BackendUserForm(validating: request.data)
            
            // Store
            var backendUser = BackendUser(form: backendUserForm)
            try backendUser.save()
     
            return Response(redirect: "/admin/backend_users").flash(.success, "User created")
        }catch FormError.validationFailed(let fieldSet) {
            return Response(redirect: "/admin/backend_users/create").flash(.error, "Validation error").withFieldset(fieldSet)
        }catch {
            return Response(redirect: "/admin/backend_users/create").flash(.error, "Failed to create user")
        }
    }

If you feel this is something you want in the package, I can make a PR next week!