luci-base: properly handle promise targets in Request.request()

Under some circumstances, ubus RPC requests may be initiated while LuCI is
still resolving the `rpcBaseURL` value. In this situation, the `target`
argument of the `request()` call will be a pending promise object which
results in an invalid URL when serialized by `expandURL()`, leading to an
`Failed to execute 'open' on 'XMLHttpRequest': Invalid URL` exception.

This commonly occured on the index status page which immediately initiates
ubus RPC calls on load to discover existing status page partials.

Solve the issue by filtering the given `target` argument through
`Promise.resolve()` before expanding the URL and initiating the actual
request.

Fixes: #3747
Signed-off-by: Jo-Philipp Wich <jo@mein.io>