Check if there are conflicts on master when running the build

- This is taken from
  https://github.com/alphagov/specialist-publisher/commit/9a097249c142c6e7146706dfd5fc0282c92f4886,
  as a thing discussed in tech lead standup that could be useful and
  passed to me to do via Jamie.
Merge pull request #106 from alphagov/jenkins_test_merge_result_of_branches

Check if there are conflicts on master when running the build
Update db/structure.sql

* No changes directly relevant to bouncer
* Has new `host_paths.canonical_path`
Remove path_hash from bouncer tests

* Was required when it was part of a UNIQUE index
* Following deploy of $INSERT_RELEVANT_COMMIT_HERE, is not required