Static pages and not found behavior

[kosarko]

@milanmajchrak ta 404 jeste asi nefunguje dobre. zustanu viset na:

Originally posted by @kosarko in #1031

@amadulhaxxani

1. The static pages handler doesn't behave correctly on "page not found" (e.g. /static/not_there) - it should show a 404 error similar to Licenses text and other static content clarin-dspace#1031 (comment) but shows "error loading details" (the screenshot above).
2. different (if .html present) behavior when switching languages
   • /static/license-ud-2.17 (when language is english this work, when language is czech this is broken as with 1.)
   • /static/license-ud-2.17.html (works for both languages, where the czech text matches the english (that should be a fallback mechanism when the czech text is not present))
   • when this is fixed we should remove any duplicities from src/static-files/cs (anything that's just a copy of the english file)

[kosarko]

@amadulhaxxani maybe a related thing to this one - https://dspace-dev.ufal.mff.cuni.cz/repository/static/VERSION_D - works on chrome but not on firefox. VERSION_D is added during the image build