Feature/csv import improvements

[eacasu]

CSV import extensions:

• event_type can now be specified as column in csv file. If column is not present, default collectives is used.
• multiple leaders can now be specified. All columns starting with user_id will be considered for adding leaders. Leaders should be specified as "license_id" or "string(license_id)" or "license_id(string)".
• multiple tags can now be specified. All columns starting with tag will be considered for adding tags.
• delimiter is now determined with method csv.Sniffer().sniff

Potential retrocompatibility issues:

• dictionary keys are now read from first row, thus they need to be specified correctly. Previously they were dictated by app.config["CSV_COLUMNS"] and they needed to have a specific order.
• All files that did not contain correct key names in first row will not work after this merge request.