Display pipeline creation in progress in MR widget
## Problem
If a pipeline is being created in the backed, the MR pipeline widget does not immediately reflect that.
## Proposal
Make the pipeline widget in the MR react real-time with pipeline creation. We do track when a pipeline is being created for an MR now and it would be awesome if we could immediately show a message in place, that a pipeline is being created. Similarly with the merge button which takes a while to refresh.
In the frontend it would be something like:
* Check if pipeline for MR is being created - YES: display placeholder/message
* Otherwise query for the current head pipeline
issue