Use fixture sharing object feature instead of retreiving data from the database

[LucileDT]

Context

We currently select object from the database in the fixtures using their labels but the fixtures bundle provides a better way to use previously created objects: https://symfony.com/doc/master/bundles/DoctrineFixturesBundle/index.html#sharing-objects-between-fixtures

We should use this method instead as it is clearer, faster, and less prone to errors.

Todo

• update user fixtures
• update translation fixtures
• update membership fixtures
• update donation fixtures