Reported Challenge - space in account name breaks syntax

[ljdelight]

Describe the bug
When reporting a challenge, the generated GH issue does not work as expected when there is a space in account name.
For example see maproulette/challenge-reports#14 or maproulette/challenge-reports#9, the Reported By appears as Reported by: [Mateusz Konieczny](https://www.openstreetmap.org/user/Mateusz Konieczny) when it's expect to appear as 'Reported by: Mateusz Konieczny'.

The space literal in the former should have been replaced with %20, which would have matched the latter.

Expected behavior
Reported by links should work.

Initially reported here maproulette/maproulette-backend#1031