Add foreign key between ci_runner_projects and ci_runners

When comparing the database schema between ci_runner_namespaces and ci_runner_projects, it is clear that one important constraint is missing:

We should try to avoid having these edge cases between ci_runner_namespaces and ci_runner_projects, as it makes the codebase harder to follow, and requires special-case logic to be spread around in the codebase to handle the 2 different scenarios.

When a runner is deleted, there's no point for ci_runner_projects records to remain in the database, so we should have a FK in place to ensure referential integrity.

Implementation plan

1. Delete all ci_runner_projects records which don't point to a valid runner. This represents just over 30% of the records of that table in .com:

   gitlabhq_dblab> SELECT COUNT(*) FROM ci_runner_projects WHERE NOT EXISTS (SELECT 1 FROM ci_runners WHERE ci_runners.id = ci_runner_projects.runner_id)
   +--------+
   | count  |
   |--------|
   | 546619 |
   +--------+
   SELECT 1
   Time: 17.769s (17 seconds), executed in: 17.768s (17 seconds)
   gitlabhq_dblab> SELECT COUNT(*) FROM ci_runner_projects
   +---------+
   | count   |
   |---------|
   | 1790283 |
   +---------+
   SELECT 1
   Time: 1.927s (1 second), executed in: 1.925s (1 second)

2. Add FK to ci_runner_projects table:

   ALTER TABLE ci_runner_projects
     ADD CONSTRAINT fk_project_runner_id FOREIGN KEY (runner_id) REFERENCES ci_runners (id) ON DELETE CASCADE;