Allow uploading images (#13)

Adds an optional field for the post image.

Fixes #12

Co-authored-by: clfreville2 <clement.freville2@etu.uca.fr>
Reviewed-on: https://codefirst.iut.uca.fr/git/clement.freville2/herbarium/pulls/13
Reviewed-by: Bastien OLLIER <bastien.ollier@noreply.codefirst.iut.uca.fr>

config/bundles.php

@@ -18,4 +18,5 @@ return [
     Nelmio\CorsBundle\NelmioCorsBundle::class => ['all' => true],
     ApiPlatform\Symfony\Bundle\ApiPlatformBundle::class => ['all' => true],
     DAMA\DoctrineTestBundle\DAMADoctrineTestBundle::class => ['test' => true],
+    Vich\UploaderBundle\VichUploaderBundle::class => ['all' => true],
 ];