Merge request review size metric

Description

As a reviewer I would like to know how large is a merge request that I have assigned to review.

Currently we need to click Changes to see number of lines added / removed but this is far from perfect.

I don't want to wait until diffs are loaded which can take a significant amount of time when a merge request is large
Lines added / removed metric is not accurate because 100 of markdown docs is much easier to review than 100 lines of complex Ruby code

Make it easier to see merge request review size.
Consider creating a little more complex metric, maybe design a gem for that (not in the first iteration for sure).
Consider making it possible to disallow merging a merge request when it greatly exceeds reasonable review size.

Make lines added / changed easier to find and see (UX). Maybe sum those two values into one, to provide single value.

This idea comes from UGC session from GitLab Summit in Crete!