Use absinthe query to find a vote by user email & proposal url

[davefrey]

In api_client.rb there is a private method user_voted?(participant_email, proposal_url) that currently retrieves all LV votes and then filters locally on mail and url.

Instead we need a parameterized query to just return the vote(s) of interest. Once this is repaired, we can probably pull up the private into the public method used to expose the call. The api-side of this problem is in issue liquidvotingio/api#230.

This same exact problem for finding a delegate is in issue #21 (api_client.rb) and liquidvotingio/api#163 (api side).