Make use of overwriteable editURL feature of Hugo Relearn

[cagix]

Hugo Relearn enhanced the editButton feature (McShelby/hugo-theme-relearn#762). For each page an individual variable editURL can be set to overwrite the global editURL configuration and to point to a custom edit page.

In pre-processing, the Lua filter needs so store the old path of a file before "warping". this needs to go as variable definition into the makefile, and be set as metavariable in the pandocmd-to-hugomd step, thus arriving in the yaml header of each file ...

scheduled for https://github.com/McShelby/hugo-theme-relearn/milestone/133