CI/CD: Job refactoring
    
    Following up https://pagure.io/waiverdb/issue/263.
    
    1. Split `waiverdb-dev` job into `waiverdb-premerge` and `waiverdb-postmerge`
    for better readability. The new 2 jobs are generated from the same template `waiverdb-build`.
    2. Move Pagure pull-request flagging and mail sending code from polling job to `waiverdb-premerge` and `waiverdb-postmerge` jobs.
    3. Use [C3I Library][1] to interact with Pagure API and wait for downstream builds.
    
    [1]: https://pagure.io/c3i-library
    
    optimize