Add absinthe query to find a delegation by delegator email & proposal url

[jinjagit]

See #19 for details of why this is useful.

TLDR: This query is useful in our view logic (and probably elsewhere) in our decidim-liquidvoting work.

This is related to #21 - the decidim-liquidvoting side of this enhancement.

[oliverbarnes]

Will need a companion issue on ruby-client repo as well