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

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 https://github.com/liquidvotingio/api/issues/230.

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

Start in api_client.rb at the private user_voted?(participant_email, proposal_url) method, then review API issue 230 for the server-side query requirements. Replace the local filtering of all LV votes with a parameterized query that returns only votes matching the email and proposal URL, and confirm the existing public call still exposes the expected result.

Written by the indexing model from the issue text.