Added bookshelves to breadcrumbs

- Updated breadcrumb dropdown switchers and back-end sibling code to handle new breadcrumbs.
- Added breadcrumb view composer and EntityContext system to mangage
tracking if in the context of a bookshelf.

app/Entities/EntityContextManager.php

@@ -0,0 +1,62 @@
+<?php namespace BookStack\Entities;
+
+use BookStack\Entities\Repos\EntityRepo;
+use Illuminate\Session\Store;
+
+class EntityContextManager
+{
+    protected $session;
+    protected $entityRepo;
+
+    protected $KEY_SHELF_CONTEXT_ID = 'context_bookshelf_id';
+
+    /**
+     * EntityContextManager constructor.
+     * @param Store $session
+     * @param EntityRepo $entityRepo
+     */
+    public function __construct(Store $session, EntityRepo $entityRepo)
+    {
+        $this->session = $session;
+        $this->entityRepo = $entityRepo;
+    }
+
+    /**
+     * Get the current bookshelf context for the given book.
+     * @param Book $book
+     * @return Bookshelf|null
+     */
+    public function getContextualShelfForBook(Book $book)
+    {
+        $contextBookshelfId = $this->session->get($this->KEY_SHELF_CONTEXT_ID, null);
+        if (is_int($contextBookshelfId)) {
+
+            /** @var Bookshelf $shelf */
+            $shelf = $this->entityRepo->getById('bookshelf', $contextBookshelfId);
+
+            if ($shelf && $shelf->contains($book)) {
+                return $shelf;
+            }
+
+        }
+        return null;
+    }
+
+    /**
+     * Store the current contextual shelf ID.
+     * @param int $shelfId
+     */
+    public function setShelfContext(int $shelfId)
+    {
+        $this->session->put($this->KEY_SHELF_CONTEXT_ID, $shelfId);
+    }
+
+    /**
+     * Clear the session stored shelf context id.
+     */
+    public function clearShelfContext()
+    {
+        $this->session->forget($this->KEY_SHELF_CONTEXT_ID);
+    }
+
+}