Generate Assertions endpoint

[vteague]

Implement the Generate Assertions endpoint, calling the /generate-assertions endpoint in raire-service.

[vteague]

Note this will need to deal with error responses from raire-service, e.g. TIED_WINNERS. These are returned in the http header and (in most cases) should be reported to the user.

[vteague]

Also note this will have to update the winner field in the data structure subsequently used to generate reports. This was not done in the prototype. I think this is ContestResult, but need to check.

We debated whether it's appropriate to have an IRVContestResult for just this reason.

[vteague]

Use ContestResult::setWinners and ContestResult::setLosers. IRV has only one winner, so every candidate who is not the winner is a loser.
Explicitly set both to blank if an error is returned (because otherwise corla will fill them with plurality winners based on preference-mentions, which is meaningless).

[vteague]

The persistence and proper reporting of winners are now in new cards: #136, #137