Links optimization

[fredericlestage-vdm]

Liquid has a handy tag called link and that can be used like this: {% link path/to/page.html %}.

However, since we're splitting our pages in subfolders for english and french, the default link tag cannot work as is (we can't use a variable in the path).

In order to reduce build travis build failure (jekyll will not build when a link is broken when using the link tag) and also assure that our links a simply not broken, we could look into building our own tag that would essentially do the same but append the page.lang and the site.baseurl (which should already be implemented as of v4.0 and we're on v6.4.1 of Jekyll, but my tests showed that it was not the case).