Receipt files export

[micsucmed]

closes #6331

[ThiefMaster]

For the failing tests:

• I suggest skipping the url since including token URLs (which has an expiry) in the export doesn't make lots of sense
• The file paths also have a reference to the IDs, so currently there's another failure in the tests when you just run some tests, e.g. using pytest indico/modules/users/export_schemas_test.py because then less IDs have been consumed (we do not commit anything during/between tests, but auto-incrementing serial IDs are consumed regardless).

I just pushed commits fixing both these issues.