Optionally send invitations for SparkleShare

[SparkleShare](http://sparkleshare.org) is an open source dropbox like file synchroniser. It uses git as a distribution and version control system.

This pull request adds a configuration option to gitlab.yml to enable sending [sparkle share invites](https://github.com/hbons/SparkleShare/wiki/Invites) when a user is added to a project (with developer or master level). This invite adds the project's repository to the user's Sparkle Share application. It also triggers Sparkle Share to post its public ssh key to Gitlab, which links it to the user.