[Balance sheet] Attach files to a Balance sheet table entry

**Feature description: A user can add documents to a balance sheet entry**

Flow:
1. Go to Main menu > "Extrase de cont" > select a Balance sheet from the Balance sheet list / add new Balance sheet > "Intrări și ieșiri" > select an entry and click on  (more actions button) > click on "Editează" in the dropdown menu > new page "Editare linie"
2. Go to “Documente justificative” section in the form, and click on “Adaugă documente”.
3. A popup appears, where the user can drag and drop documents in order to upload them as described in issue https://github.com/code4romania/alex-api/issues/23


[Figma mockup](https://www.figma.com/file/W7ppl5xGFpB3atVWwYL7qN/Alex-(asistentul-bugetar-al-ONG-urilor)?node-id=3124%3A41738)

Actions on Add documents to a balance sheet entry:
1. Save - to save a file, a user can click on 'Salveaza' button
2. Delete - to remove a file from the list, the user can click on the X (delete button).