Fix a back-link bug

[dthex5d]

A bug, which creates the empty-back-link when the page.back_page property is not declared, has been found. It seems like the 'find' filter of the liquid does not work properly. Now, back-link.html uses both 'where' and 'first' filters to create the proper back link, only when the the page.back_page is declared in the current page.

[Tetr4]

I think the issue is, that the find filter requires Jekyll 4.1.0, but GitHub Pages currently only supports 3.9.2, so the back arrow is always shown, but has no href or label.