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/SearchTerm.php

@@ -0,0 +1,19 @@
+<?php namespace BookStack\Entities;
+
+use BookStack\Model;
+
+class SearchTerm extends Model
+{
+
+    protected $fillable = ['term', 'entity_id', 'entity_type', 'score'];
+    public $timestamps = false;
+
+    /**
+     * Get the entity that this term belongs to
+     * @return \Illuminate\Database\Eloquent\Relations\MorphTo
+     */
+    public function entity()
+    {
+        return $this->morphTo('entity');
+    }
+}