Bitbucket Pipelines

Paste integrations/ci/bitbucket/bitbucket-pipelines.yml (a pull-request preview and a main sync). Add secured repository variables RL_API_KEY, RL_API_URL and RL_SITE.

• The preview is written to the log and to the rl-preview.md artifact. No Bitbucket comment is posted: that is not implemented.
• Forked pull requests do not receive secured variables, so they get the offline count.
• Same errors and exit codes as the other CI flavours (plain ERROR: lines); see Troubleshooting CI runs.

Nothing was run on Bitbucket and the pipeline syntax is verify against Atlassian's documentation. Follow the security rules.