Github import: incorrect amount of imported issues in import status endpoint

Endpoint /projects/:id/import currently reports incorrect amount of imported issues in the stats field.

In case of rspec/rspec-mocks repository import, following stats are returned:

{
    "diff_note": 2173,
    "issue": 1459,
    "label": 7,
    "milestone": 10,
    "note": 6259,
    "pull_request": 873,
    "pull_request_merged_by": 688,
    "pull_request_review": 302,
    "release": 11
  }

Field issue includes pull requests and issues combined instead of just issues.

It should actually be 1459 - 873 = 586