Add a label about the change size of the MR

[julioleitao]

"Developers should be encouraged to keep each individual change small, with larger changes preferably broken into a series of smaller changes that a reviewer can easily review in one go.

One way in which keeping changes small is encouraged is that the code review tools label each code review with a description of the size of the change, with changes of 30-99 lines added/deleted/removed being labelled 'medium-size' and with changes of above 300 lines being labelled with increasingly disparaging labels, e.g. 'large' (300-999), 'freakin huge' (1000-1999)" Fergus Henderson at Software Engineering at Google.

We could have a label at the notification to show the impact of the MR.