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>

composer.json

@@ -46,7 +46,8 @@
         "symfony/yaml": "7.0.*",
         "symfonycasts/verify-email-bundle": "^1.17",
         "twig/extra-bundle": "^2.12|^3.0",
-        "twig/twig": "^2.12|^3.0"
+        "twig/twig": "^2.12|^3.0",
+        "vich/uploader-bundle": "^2.3"
     },
     "config": {
         "allow-plugins": {