Code review

This commit fixes some superficial instances of Broken Access Control 
(https://owasp.org/www-project-top-ten/OWASP_Top_Ten_2017/Top_10-2017_A5-Broken_Access_Control).
There may be some more instances of this, as authorization was only done 
after most of the controllers were done (big mistake).

Some refactoring was also performed, where Route Model Binding with DI 
(dependency injection) was used whenever possible, to increase 
testability of the codebase.
Some reused code was also moved to Helper classes as to enforce DRY; 
There may be some lines of code that are still copy-pasted from other 
parts of the codebase for reuse.

Non-breaking refactoring changes were made, but the app as a whole still 
needs full manual testing, and customised responses to HTTP 500 
responses. Some errors are also not handled gracefully and this wasn't 
checked in this commit.

app/Notifications/NewApplicant.php

@@ -55,7 +55,7 @@ class NewApplicant extends Notification implements ShouldQueue
                     ->subject(config('app.name') . ' - New application')
                     ->line('Someone has just applied for a position. Check it out!')
                     ->line('You are receiving this because you\'re a staff member at ' . config('app.name') . '.')
-                    ->action('View Application', url(route('showUserApp', ['id' => $this->application->id])))
+                    ->action('View Application', url(route('showUserApp', ['application' => $this->application->id])))
                     ->line('Thank you!');
     }
 
@@ -67,7 +67,7 @@ class NewApplicant extends Notification implements ShouldQueue
         $vacancyDetails['name'] = $this->vacancy->vacancyName;
         $vacancyDetails['slots'] = $this->vacancy->vacancyCount;
 
-        $url = route('showUserApp', ['id' => $this->application->id]);
+        $url = route('showUserApp', ['application' => $this->application->id]);
         $applicant = $this->application->user->name;
 
         return (new SlackMessage)