A dossier has several submission dates:
- `depose_at` is the first date at which the dossier was submitted
- `en_construction_at` is the last date at which the dossier was
submitted OR reverted from `en_instruction` to `en_construction`.
For now we prefer to display the earliest date:
- it gives more rights to the user,
- en_construction_at means the administration can reset at will the date
printed on deposit receipts (by moving the dossier `en_instruction`
and then back), which we don't want.
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