This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.
Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction
Due to what seems to be a bug in ActiveRecord, the distinct: true option
is not recognized on counts with string column names. This commit fixes
that by moving the DISTINCT into the count string.
For robustness, the integration spec for IncomingLinksReport was
rewritten to be an actual integration spec, running the actual interface
on actual fake data.
