make ikiwiki show proper headings

Our documentation pages are getting increasingly long in the ikiwiki. So I sometimes need to link to specific sections. For example, this link:

https://help.torproject.org/tsa/howto/ganeti/#index4h2

currently points at "Adding a new instance". Yet there are a few problems with that link:

1. it's not human-readable: it's hard to understand what it refers to just by looking at it
2. it's not stable: if another heading is added or remove before it, its number is going to change and the link will point elsewhere

There's a plugin called headinganchors which adds better identifiers, but it has some limitations with non-ASCII characters. I wrote a replacement plugin but it's not shipped in ikiwiki by default.

I'm not "just doing this" right now because it's friday and I don't want to break the wiki before the weekend.