Normalize syntax for dates

### Describe the problem that you'd like solved
Using the API, dates (such as 'publish_date') are given without any common syntax, making it difficult to parse and display it in another language. Such fields would be much more usefull if filled with a fixed syntax.

### Proposal & Constraints
When creating/editing a book, use a date form which allows to set year, month+year or day+month+year. Wikidata is very smart on this point but no need to go this far. If needed (cases where dates are not real dates?) a checkbox can be added to display a regular text input (like today). But if we can avoid that it would be better.

Using the API, date fields should be rendered using an agreed syntax (like YYYY-MM-DD for instance). If the checkbox exists to set free dates, then it would be the entered string.

### Additional context
N/A

### Stakeholders
N/A