Common ranking

[poelstra]

Part of #245: Move Ranking computation to common code (used by ng-scores or a new ng-ranking service)

All tests pass for each commit.

The first few commits are only refactors, no functional changes.
The last 2 fix two small bugs.

To test:

• For the first commits: check that the rankings still work in the frontend
• For the negative round bug: edit a score to have a round of 0 or lower, verify that it no longer ends up in rankings
• For the crash in CSV export bug: edit a score to have a non-existing team number, verify that there are no errors in JS Console, and Export button on Ranking screen works (team info will be empty for that score, though)

[coveralls]

Changes Unknown when pulling 3c26de4 on common_ranking into ** on master**.

[kmeesters]

Test (npm test) and through the frond-end. (win 10, node 8.6.0). Works okay.
Questions though is what to do with scores with incorrect team-numbers, should they disappear from ranking like when the round-number is correct? I.e. prohibit publishing of scores if there is an error? Or should we not force it? However will make an issue for this, for now the current functionality is fine.