Renames the relation between the Pipeline and Bridge job to be more
descriptive of the intent.

This includes renaming the model relations, database columns as well as
service / worker class names.

It is now possible to pass variables to a downstream pipeline from a
bridge job. Variables defined as YAML variables for this bridge job are
going to be populated downstream, and are going to be available in every
job created within a downstream pipeline. These variables are then going
to be passed to GitLab Runner as environment variables.

Signed-off-by: Rémy Coutable <remy@rymai.me>