Added global config setting to change number of commits to display per event

Previously, the dashboard would display at most 2 commits per push event, with a link to see more ("... and N more commits").  I found the default limiting for my team's workflow, so wanted to increase it.

This is a fairly trivial change - it adds a new gitlab.yml setting:

    commits_to_display_per_event: 2

That setting defaults to 2 if not declared.