Added middleware

[JVT038]

While the name of this branch is 'add-web-authentication-middleware', this PR also adds a bunch of other middleware outside of authentication (such as checking whether the user has a Jellyfin / Plex token or is an Admin, etc.)

The middleware stuff is in src/HttpController/Middleware.

The new syntax for a route is as follows:
$routeCollector->addRoute('GET', '<URI>', [[<Class\SomeHandler::class>, '<someMethod>'], 'middleware' => [<someMiddleware::class>,<someOtherMiddleware::class>]]);.

I'll try and see if I can improve the syntax, because it's currently quite ugly IMO. If no middleware is necessary, the old syntax can be used fine; so then the handler shouldn't be in an array.

[JVT038]

Also, I made some new files in the Service directory; that's my attempt at extending the Fastroute library and trying to improve the syntax with some addMiddleware method. It can be ignored for now, because it doesn't work at all (and it's not used either).

[JVT038]

So I managed to improve the syntax a bit. This is how it currently looks like:

$routes->createNewRoute('GET', '/', [Some\Controller::class, 'callbackfunction'])->addMiddleware(Middleware\SomeMiddleware::class, Middleware\SomeOtherMiddleware::class);

[JVT038]

I'm not sure if we should replace this check with a middleware:

        $userId = $this->userPageAuthorizationChecker->findUserIdIfCurrentVisitorIsAllowedToSeeUser((string)$request->getRouteParameters()['username']);
        if ($userId === null) {
            return Response::createNotFound();
        }

The $userId variable is used often and if we were going to replace this with a middleware, the controller would still need the userId and the check method findUserIdIfCurrentVisitorIsAllowedToSeeUser() won't be removed. Wouldn't it be unnecessary to replace this thing?