[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
addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile