Add an embedded mode for platal.

Add an embedded mode for platal.

You can query /embedded/{mode}/path/to/url to get a mode in which the
skinning mode information is propagated through pages.

e.g.: https://www.p.org/embedded/light/search can be used to get the
search page with light skinning.

Note: This only requires a hack in .htaccess to work with static files.

Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>