Enable handling redirects in openfoodfacts-web

[alexgarel]

Problem

In openfoodfacts-web, symlinks enable pointing some urls to same content.

Sometime it's better to have redirects, for url that changed for example.

We have a lot of redirects right now directly in the nginx configuration, (see https://github.com/openfoodfacts/search-a-licious/)

It would be better if all this can be handled naturally in openfoodfacts-web.

Proposed solution

Handle this case by looking at files in openfoodfacts-web.

I think the best would be to have a file lang/texts/<lc>/<location>.redirect and inside put a URI to redirect to.

As we do for html files, at startup, we should gather those redirects, so that in display content function we are able to issue the right redirect.