The `Pipeline#merge_request?` is confusing

I find it strange that all our logic is based on Ci::Pipeline#source the merge_request? source event.

I propose to have consistent information that this is Merge Request pipeline, and ignore source event:

1. rename source:merge_request to source:merge_request_event,
2. add merge_request? that is merge_Request_id.present?

This way, we have merge_request? that is based on attached merge request, and we have clear information that it was created from merge request event.

cc @dosuken123