Add partition_id to Ci::PipelineMessage

Ref: #461547 (closed)

What does this MR do and why?

This MR cascades partition_id from Ci::Pipeline to Ci::PipelineMessage.

We need to add this new column in order to redefine the FK association between ci_pipeline_messages and ci_pipelines.

We are setting the value to 100 as this is the first partition value we are using on gitlab.com.

TABLE "ci_pipeline_messages" CONSTRAINT "fk_rails_8d3b04e3e1" FOREIGN KEY (pipeline_id) REFERENCES ci_pipelines(id) ON DELETE CASCADE

This is a similar approach than we used for other CI models in #430722 (closed).