Make the reproducer mandatory in the issue template

Summary:
As the title says, let's make this field mandatory so we don't have to ask for
a repro over and over again.

Changelog:
[Internal] [Changed] - Make the reproducer mandatory in the issue template

Reviewed By: cipolleschi

Differential Revision: D36552021

fbshipit-source-id: ac6aa0c3b1583afe38b3fcf78c385e5bdfb5f6e2