Repair the GraphViz feature

[hangy]

What

As I discovered in #1458, the exported GEXF graph is kind of outdated and limited to French only. Since they are generated by export_tags_hierarchy, too, the SVG and PNG graphs probably have not been re-created for a while, either.

We should review if this is a feature that we want to support going forward. If we want to keep it, we can use this issue to track what is required to re-enable it and how to enable it for more languages. Otherwise, let's track the removal in here.

Notes: This is the only instance where GraphViz2 is required by ProductOpener. Removing this would allow us to remove a deployment dependency. Also, Sigma.js provides an alternative SVG export.

Part of

• Power tools

[stephanegigandet]

It is a very old (2013) feature and graph, but I'd like to keep it (and repair it).