Improve client performance by enabling HTTP caching for i18n REST resources

REST i18n resources are generally derived from static information inside plugins and are unlikely to change very often. We should use our short-term public caching headers for them instead of not specifying any caching headers.

That is, we need to change these lines:

httpContext.getResponse().setHeader("Cache-Control", "");
httpContext.getResponse().setHeader("Pragma", "");
httpContext.getResponse().setHeader("Expires", "");

to this:

CachingHeaders.PUBLIC_SHORT_TERM.apply(httpContext.getResponse());

In addition, we should probably configure the REST caching consistently in our central caching filter, rather than having custom handling in the plugin.