Moved models to folder, renamed managers to tools

Tools seems to fit better since the classes were a bit of a mixed bunch
and did not always manage.
Also simplified the structure of the SlugGenerator class.
Also focused EntityContext on shelves and simplified to use session
helper.

app/Entities/Tools/ShelfContext.php

@@ -0,0 +1,42 @@
+<?php namespace BookStack\Entities\Tools;
+
+use BookStack\Entities\Book;
+use BookStack\Entities\Bookshelf;
+
+class ShelfContext
+{
+    protected $KEY_SHELF_CONTEXT_ID = 'context_bookshelf_id';
+
+    /**
+     * Get the current bookshelf context for the given book.
+     */
+    public function getContextualShelfForBook(Book $book): ?Bookshelf
+    {
+        $contextBookshelfId = session()->get($this->KEY_SHELF_CONTEXT_ID, null);
+
+        if (!is_int($contextBookshelfId)) {
+            return null;
+        }
+
+        $shelf = Bookshelf::visible()->find($contextBookshelfId);
+        $shelfContainsBook = $shelf && $shelf->contains($book);
+
+        return $shelfContainsBook ? $shelf : null;
+    }
+
+    /**
+     * Store the current contextual shelf ID.
+     */
+    public function setShelfContext(int $shelfId)
+    {
+        session()->put($this->KEY_SHELF_CONTEXT_ID, $shelfId);
+    }
+
+    /**
+     * Clear the session stored shelf context id.
+     */
+    public function clearShelfContext()
+    {
+        session()->forget($this->KEY_SHELF_CONTEXT_ID);
+    }
+}