Export fails with certain symbols

[olligobber]

If a proof contains a symbol like #, exporting the proof fails.

This can be replicated by adding a symbol alias from # to any other symbol, and then exporting the proof. The downloaded json file simply ends where the # would normally be.

This is likely due to the way files are downloaded by creating a data url on the fly, in which a # and possibly other characters are interpreted as part of the url rather than part of the data.