GSoC milestone tracking

As a yearly event, we need to build systems that streamline our GSoC processes and systems. We currently use GitLab repositories for tracking milestones.

We have https://gitlab.com/coala/GSoC-2017/ and https://gitlab.com/coala/GSoC2016, with slightly different repository names. We should use a consistent naming convention so that these gitlab tracking repos can be easily accessed by other systems. e.g. https://github.com/coala/community/issues/59

And next year, will we create another repo? Or do we rename the 2017 repo to a generic name , or create a generic "GSoC tracking" repo for all future gsoc? @nkprince007 found [issue moving](https://docs.gitlab.com/ee/api/issues.html#move-an-issue) may be an option (however based on https://gitlab.com/gitlab-org/gitlab-ce/issues/20372#note_29025219 it would be wise to test moving issues first, to make sure the original author & date is retained)

I find one repo per year to be undesirable, as each repo wont grow beyond that short period of time, so there is little inertia and motivation to invest time into it beyond its limited purpose.

The 2017 repo was abandoned after gsoc until I cleaned it up before GCI.

See https://gitlab.com/coala/GSoC2016/issues/89 for more info about that one. It is likewise abandoned.