Some pages use relative links such as boards link at the home page.
Others use absolute url such as cards in boards' lists.
This commits goal is to allow for consistent use of relative
urls.
Origin relative URLs also helps decoupling Wekan from the infrastructure
it's deployed on. i.e if it's being served, it should work.
Bug Fix#2093: the broken should be prior to file attachment feature introduced, and tested export board is working.
Thanks to whowillcare !
( xet7 merged this pull request manually from https://github.com/wekan/wekan/pull/2756 )
Closes#2093