When gitlab_rails['enable'] = false, the database won't be created for
the load to happen, causing an error from the
postgresql_extension[pg_trgm] resource:

  postgresql_extension[pg_trgm] (gitlab::postgresql line 213) had an error: Mixlib::ShellOut::ShellCommandFailed: postgresql_query[enable pg_trgm extension] (/opt/gitlab/embedded/cookbooks/cache/cookbooks/postgresql/resources/extension.rb line 6) had an error: Mixlib::ShellOut::ShellCommandFailed: execute[enable pg_trgm extension (gitlab-psql)] (/opt/gitlab/embedded/cookbooks/cache/cookbooks/postgresql/resources/query.rb line 11) had an error: Mixlib::ShellOut::ShellCommandFailed: Expected process to exit with [0], but received '2'
  ---- Begin output of /opt/gitlab/bin/gitlab-psql -d gitlabhq_production -c "CREATE EXTENSION IF NOT EXISTS pg_trgm" ----
  STDOUT:
  STDERR: psql: FATAL:  database "gitlabhq_production" does not exist
  ---- End output of /opt/gitlab/bin/gitlab-psql -d gitlabhq_production -c "CREATE EXTENSION IF NOT EXISTS pg_trgm" ----
  Ran /opt/gitlab/bin/gitlab-psql -d gitlabhq_production -c "CREATE EXTENSION IF NOT EXISTS pg_trgm" returned 2

Looking at the postgresql_extension LWRP we see that the logic around
whether to enable the extension is a quite hairy not_if expression,
which is hard to follow and debug.  To improve this here we extract the
existing BasePgHelper#extension_can_be_enabled? and use that to simplify
the flow of the `postgresql_extension` LWRP and `recipes/postgresql_spec`