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/PageEditActivity.php

@@ -0,0 +1,74 @@
+<?php namespace BookStack\Entities\Tools;
+
+use BookStack\Entities\Page;
+use BookStack\Entities\PageRevision;
+use Carbon\Carbon;
+use Illuminate\Database\Eloquent\Builder;
+
+class PageEditActivity
+{
+
+    protected $page;
+
+    /**
+     * PageEditActivity constructor.
+     */
+    public function __construct(Page $page)
+    {
+        $this->page = $page;
+    }
+
+    /**
+     * Check if there's active editing being performed on this page.
+     * @return bool
+     */
+    public function hasActiveEditing(): bool
+    {
+        return $this->activePageEditingQuery(60)->count() > 0;
+    }
+
+    /**
+     * Get a notification message concerning the editing activity on the page.
+     */
+    public function activeEditingMessage(): string
+    {
+        $pageDraftEdits = $this->activePageEditingQuery(60)->get();
+        $count = $pageDraftEdits->count();
+
+        $userMessage = $count > 1 ? trans('entities.pages_draft_edit_active.start_a', ['count' => $count]): trans('entities.pages_draft_edit_active.start_b', ['userName' => $pageDraftEdits->first()->createdBy->name]);
+        $timeMessage = trans('entities.pages_draft_edit_active.time_b', ['minCount'=> 60]);
+        return trans('entities.pages_draft_edit_active.message', ['start' => $userMessage, 'time' => $timeMessage]);
+    }
+
+    /**
+     * Get the message to show when the user will be editing one of their drafts.
+     * @param PageRevision $draft
+     * @return string
+     */
+    public function getEditingActiveDraftMessage(PageRevision $draft): string
+    {
+        $message = trans('entities.pages_editing_draft_notification', ['timeDiff' => $draft->updated_at->diffForHumans()]);
+        if ($draft->page->updated_at->timestamp <= $draft->updated_at->timestamp) {
+            return $message;
+        }
+        return $message . "\n" . trans('entities.pages_draft_edited_notification');
+    }
+
+    /**
+     * A query to check for active update drafts on a particular page
+     * within the last given many minutes.
+     */
+    protected function activePageEditingQuery(int $withinMinutes): Builder
+    {
+        $checkTime = Carbon::now()->subMinutes($withinMinutes);
+        $query = PageRevision::query()
+            ->where('type', '=', 'update_draft')
+            ->where('page_id', '=', $this->page->id)
+            ->where('updated_at', '>', $this->page->updated_at)
+            ->where('created_by', '!=', user()->id)
+            ->where('updated_at', '>=', $checkTime)
+            ->with('createdBy');
+
+        return $query;
+    }
+}