Respect namespace_id in fogbugz importer

What does this MR do and why?

Related to Importers - Default to the current group when i... (#359130 - closed)

This MR correctly reuses namespace_id through import process of fogbugz importer. This allows us to correctly select current group as default for import.

Screenshots or screen recordings

Flightjs___GitLab__2_

How to set up and validate locally

1. Visit any group
2. New project -> Import -> FogBugz
3. Use http://importtesting2.fogbugz.com/ as url and iklymov+fogbugz@gitlab.com / testing as credentials
4. Just use default usermap
5. Observe that group selected as default import

MR acceptance checklist

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