[backend][urls] Add + sign in environment pattern url

This allows to have C++ as environment name.

Fixes #556
1 job for 556_cannot_show_environment_with_plus_sign_in_name in 14 minutes and 14 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #205208
docker-build
build_linux_36

00:14:14

65.0%