Refactor Flarum\Web and Flarum\Admin

- In order to be consistent with the Ember/LESS naming scheme, renamed
Flarum\Web to Flarum\Forum.
- Moved common classes into Flarum\Support so that Flarum\Admin doesn’t
depend on Flarum\Forum. Also moved Actor into Flarum\Support as it
doesn’t belong in the domain.

src/Core/Support/Actor.php

@@ -1,23 +0,0 @@
-<?php namespace Flarum\Core\Support;
-
-use Flarum\Core\Models\Guest;
-
-class Actor
-{
-    protected $user;
-
-    public function getUser()
-    {
-        return $this->user ?: new Guest;
-    }
-
-    public function setUser($user)
-    {
-        $this->user = $user;
-    }
-
-    public function isAuthenticated()
-    {
-        return (bool) $this->user;
-    }
-}