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/Models/PageRevision.php

@@ -0,0 +1,84 @@
+<?php namespace BookStack\Entities;
+
+use BookStack\Auth\User;
+use BookStack\Model;
+use Carbon\Carbon;
+
+/**
+ * Class PageRevision
+ * @property int $page_id
+ * @property string $slug
+ * @property string $book_slug
+ * @property int $created_by
+ * @property Carbon $created_at
+ * @property string $type
+ * @property string $summary
+ * @property string $markdown
+ * @property string $html
+ * @property int $revision_number
+ */
+class PageRevision extends Model
+{
+    protected $fillable = ['name', 'html', 'text', 'markdown', 'summary'];
+
+    /**
+     * Get the user that created the page revision
+     * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
+     */
+    public function createdBy()
+    {
+        return $this->belongsTo(User::class, 'created_by');
+    }
+
+    /**
+     * Get the page this revision originates from.
+     * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
+     */
+    public function page()
+    {
+        return $this->belongsTo(Page::class);
+    }
+
+    /**
+     * Get the url for this revision.
+     * @param null|string $path
+     * @return string
+     */
+    public function getUrl($path = null)
+    {
+        $url = $this->page->getUrl() . '/revisions/' . $this->id;
+        if ($path) {
+            return $url . '/' . trim($path, '/');
+        }
+        return $url;
+    }
+
+    /**
+     * Get the previous revision for the same page if existing
+     * @return \BookStack\Entities\PageRevision|null
+     */
+    public function getPrevious()
+    {
+        $id = static::newQuery()->where('page_id', '=', $this->page_id)
+            ->where('id', '<', $this->id)
+            ->max('id');
+
+        if ($id) {
+            return static::query()->find($id);
+        }
+
+        return null;
+    }
+
+    /**
+     * Allows checking of the exact class, Used to check entity type.
+     * Included here to align with entities in similar use cases.
+     * (Yup, Bit of an awkward hack)
+     * @param $type
+     * @return bool
+     */
+    public static function isA($type)
+    {
+        return $type === 'revision';
+    }
+}