luci-static: fix race condition when probing features

follow-up to a2fce95248

hasSystemFeature() inspects the cached result of
probeSystemFeatures() which needs to have completed.

Once all of the legacy (Lua) stuff is ready, this block
runs:

initDOM() {
	...
	L.loaded = true;
	document.dispatchEvent(new CustomEvent('luci-loaded'));
},

This commit now calls View.load() once system feature
probing has completed, and LuCI itself has loaded. This
ensures that the following load paradigm succeeds:

return view.extend({
	load() {
		return Promise.all([
			...,
			L.hasSystemFeature('x'),
		]);
	}, ...

The luci-loaded check prevents waiting when LuCI is already
loaded (the common case after initial page load), but the
listener handles the race condition where a View is
instantiated before initDOM() completes.

The flow:

If L.loaded is true: initDOM() already ran --> skip waiting,
proceed immediately.

If L.loaded is false: initDOM() hasn't run yet --> add
listener --> wait for it to complete.

Signed-off-by: Paul Donald <newtwen+github@gmail.com>

modules/luci-base/htdocs/luci-static/resources/luci.js

@@ -1899,16 +1899,14 @@
 
 			DOM.content(vp, E('div', { 'class': 'spinning' }, _('Loading view…')));
 
-			return Promise.resolve(this.load())
-				.then(function (...args) {
-					if (L.loaded) {
-						return Promise.resolve(...args);
-					} else {
-						return new Promise(function (resolve) {
-							document.addEventListener('luci-loaded', resolve.bind(null, ...args), { once: true });
-						});
-					}
-				})
+			const ready = L.loaded
+				? Promise.resolve()
+				: new Promise((resolve) => {
+					document.addEventListener('luci-loaded', resolve, { once: true });
+				});
+
+			return ready
+				.then(LuCI.prototype.bind(this.load, this))
 				.then(LuCI.prototype.bind(this.render, this))
 				.then(LuCI.prototype.bind(function(nodes) {
 					const vp = document.getElementById('view');