Add .tpl for rendering broken wikilinks, so the user can override it

[kksgandhi]

Discussed in #220

^{Originally posted by kksgandhi December 31, 2021}
At the moment, a broken wikilink has a strikethrough effect, naked brackets, a large X, and a "backlinks" superscript link. This seems rather excessive, and I was wondering if there was a user controllable way to set what broken wikilinks should look like? Furthermore, is there a way to create a template for the "broken link, here's the backlinks" page?