Convert tests/test_elsewhere_github.py to use Harness & TestClient.

Relates to #540
Convert tests/test_elsewhere_twitter.py to use Harness & TestClient.

Relates to #540.
Trim & convert tests/test_is_suspicious.py to use Harness & TestClient.

Relates to #540
Convert tests/test_pages.py to use Harness & TestClient.

Relates to #540.
Convert tests/test_participant.py to use Harness.

Relates to #540.
Convert tests/test_stats.py to use Harness & TestClient.

Relates to #540.
Convert tests/test_billing{_payday}.py to use Harness.

Relates to #540.
Correct imports
Removed incorrect test
Merge pull request #591 from joonas/convert-tests-to-harness

Convert tests to use Harness, fixes #540