Convert Askama templates to Tera

[Weasy666]

Askama is a great template engine, but templates are compiled into the binary and as such, it is not possible to change them at runtime. But we want to eventually use templates for mail notifications and want them to be customizable. Because of that, the templates need to be ported to Tera or Ramhorns, which make it possible to change templates without the need to recompile everything.