Fix CSV upload without timestamp_desc

[jkppr]

The timestamp_desc field is a mandatory field for all Timesketch data. However, when uploading via the web UI the upload form did not trigger the mapping functionality when the timestamp_desc field was missing. This resulted in a broken timeline with the error:

Error message: Missing mandatory CSV headers.
Mandatory headers: datetime, message, timestamp_desc
Headers found in the file: datetime, timestamp, packages, message, command, display_name, timestamp_descA, parser, filename, data_type, extra_field_1
Headers provided in the mapping: None
Headers missing: timestamp_desc

This PR adds timestamp_desc to the mandatory fields wich will therefore trigger the mapping feature in the upload dialog if the field is missing.

Closing issues

closes #2630