Changes all calls to data_source_exists? to table_exists?
since that is the intent of these calls

We also enable the rubocop that makes it mandatory

Instead of inserting a row after each example to an external database,
we save the CI profiling reports into the `rspec_profiling` directory
and insert the data in the update-tests-metadata CI stage.  This should
make each spec run faster and also reduce the number of PostgreSQL
connections needed by concurrent CI builds.

`scripts/insert-rspec-profiling-data` also inserts one file at a time
via the PostgreSQL COPY command for faster inserts. The one side effect
is that the `created_at` and `updated_at` timestamps aren't available
since they aren't generated in the CSV.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/10154