Apply fixes from StyleCI (#332)

Author: driesvints

app/Exceptions/CannotCreateUser.php

@@ -6,12 +6,12 @@
 
 class CannotCreateUser extends Exception
 {
-    public static function duplicateEmailAddress(string $emailAddress): CannotCreateUser
+    public static function duplicateEmailAddress(string $emailAddress): self
     {
         return new static("The email address [$emailAddress] already exists.");
     }
 
-    public static function duplicateUsername(string $username): CannotCreateUser
+    public static function duplicateUsername(string $username): self
     {
         return new static("The username [$username] already exists.");
     }

app/User.php

@@ -216,17 +216,17 @@ public function countSolutions(): int
         })->count();
     }
 
-    public static function findByUsername(string $username): User
+    public static function findByUsername(string $username): self
     {
         return static::where('username', $username)->firstOrFail();
     }
 
-    public static function findByEmailAddress(string $emailAddress): User
+    public static function findByEmailAddress(string $emailAddress): self
     {
         return static::where('email', $emailAddress)->firstOrFail();
     }
 
-    public static function findByGithubId(string $githubId): User
+    public static function findByGithubId(string $githubId): self
     {
         return static::where('github_id', $githubId)->firstOrFail();
     }