Fix invalid URL in admin/projects/list.html.

If the project_list contains a project that is not associated with a
team (such as "Sentry (Internal)"), then URL generation for the
sentry-stream and sentry-manage-project URLs breaks.  The "fix" is to
not emit the URLs that we cannot possibly generate without a team slug.
Merge pull request #950 from dave-shawley/fix-admin-template-internal-team

Fix invalid URL in admin/projects/list.html.