luci-mod-status: change "Storage usage" heading to just "Storage"

Also rename `25_diskfree.js` to `25_storage.js` to make naming anologeous
with "Memory" / `20_memory.js`.

Fixes: f09920f2

 ("luci-mod-status: add diskfree info")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>