luci-base: package update based cache-busting string

luci-base: package update based cache-busting string

Problem:
After updating the JS-part of any package, the user is forced to clear
the browser cache.

Fix:
Each time a package is removed or installed, the contents of the
"/lib/apk/db/installed" file are changed.
This fix add the modification time of the "/lib/apk/db/installed" file
to the version of the main LuCI package (for each js-file).

Signed-off-by: Oleg S <[email protected]>