Cf. 38931205

Also account for absolute/relative paths

Ref: bef7e6dc

We're using checksum as file name and verify that all
externally-controllable path components are harmless.

Requested by korfuri. If people complain maybe make it a preference.

Login still works regardless of case!

We don't want the "canceled stage" to be filled with talks we've never
even seen before.

empty strings aren't truthy.

Required by the more languages team.

This should really be a property of the conference, will have to clean
up later.