[req,web] Make it possible to handle page language from URL prefix Adding a short language prefix to URL (like "/en" or "/fr") changes the language the pages are displayed in. This prefix is kept during navigation. This way it is not necessary to do language negotiation, nor to use user preferences to determine which language to apply. This behavior is controlled through a new configuration option "language-mode", which replaces "language-negociation" option and which values may be "http-negotiation", "url-prefix" or "" (to disable language setting and force using "ui.language" property). Migration from previous option is not handled because I could not manage to have it working (users will get prompted with the configuration file diff anyways). Add some tests checking various scenarios. Closes #15743487

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