Add EditGroups support

[wetneb]

To make it easier to undo uploads, it could make sense to track uploads made via Pattypan with EditGroups. There is an instance of this tool running at https://editgroups-commons.toolforge.org.

It is quite easy to add support for this on Pattypan's side: all it takes is to add a link in the edit summary, as described here:
https://commons.wikimedia.org/wiki/Commons:Edit_groups/Adding_a_tool#For_custom_bots

I am suggesting this while acknowledging that EditGroups is (in my opinion) a temporary and unreliable measure to mitigate the lack of support for edit groups (or "changesets", to use the OSM term) in MediaWiki itself (or as an extension).

[Abbe98]

We should definitely implement edit groups. Thank you for such detailed documentation/code examples!