luci-base: dispatcher.uc: update uci session ID in Lua context

Make sure that the uci session ID of the `luci.model.uci` module within
the Lua context is updated once we acquire the login session information.

In case legacy themes are used, the probing of the theme header template
might indirectly load the Lua runtime and the Lua side `luci.dispatcher`
module which in turn will load the `luci.model.uci` and set the session
ID there which is not yet initialized at this point in time.

This results in broken uci change handling within legacy Lua applications
when a legacy theme is loaded.

Fixes: #6060
Signed-off-by: Jo-Philipp Wich <jo@mein.io>