Add WG abbreviation to issue template

[fantasai]

It would be helpful, I think, if we added something like (WGABBR) to the issue templates, to make it easy and reliable to search for e.g. all the issues from CSSWG.

[plehegar]

We now have labels in our horizontal repositories to track groups, eg https://github.com/w3cping/tracking-issues/issues?q=is%3Aissue+is%3Aopen+label%3Awg%3Avc

So, I'm wondering if, instead of asking people to add the information in the title, we use a tool to automatically add those labels.