luci-base: cbi.js: add client side translation infrastructure

Implement the string hash algorithm used by LuCI's translation system in
JavaScript and provide a `_()` translation wrapper function to lookup
messages in the global string table.

Once client side translation loading is activated in a later commit,
JavaScript code can use the same string translation mechanism as server
side Lua code, e.g. `_("Static Routes")` would yield "Statische Routen"
when the German translation is loaded.

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