chore: catch error when using Request, and make remote URL working again

I decided to remove the check `is_ajax` from `validate_url` to simplify
and edge case, and because I think it was more or less useless.
Basically, when getting remote data, we have two cases:
- direct call to the remote URL
- proxy through our `ajax_proxy` system (to work around CORS limitations)

In the first case, we cannot set the `X-Requested-With` header, otherwise
preflight step will fail, and in the second case, until now, we needed
to set this header for this `is_ajax` check to pass. So keeping this check
would mean adapting the behaviour of the Request/ServerRequest class in
a non elegant way. So let's make it simple…

umap/static/umap/js/umap.layer.js

@@ -747,15 +747,18 @@ L.U.DataLayer = L.Evented.extend({
     if (!this.options.remoteData.dynamic && this.hasDataLoaded() && !force) return
     if (!this.isVisible()) return
     let url = this.map.localizeUrl(this.options.remoteData.url)
-    if (this.options.remoteData.proxy)
+    if (this.options.remoteData.proxy) {
       url = this.map.proxyUrl(url, this.options.remoteData.ttl)
+    }
     const response = await this.map.request.get(url)
-    this.clear()
-    this.rawToGeoJSON(
-      await response.text(),
-      this.options.remoteData.format,
-      (geojson) => this.fromGeoJSON(geojson)
-    )
+    if (response && response.ok) {
+      this.clear()
+      this.rawToGeoJSON(
+        await response.text(),
+        this.options.remoteData.format,
+        (geojson) => this.fromGeoJSON(geojson)
+      )
+    }
   },
 
   onceLoaded: function (callback, context) {
@@ -1060,7 +1063,9 @@ L.U.DataLayer = L.Evented.extend({
   importFromUrl: async function (uri, type) {
     uri = this.map.localizeUrl(uri)
     const response = await this.map.request.get(uri)
-    this.importRaw(await response.text(), type)
+    if (response && response.ok) {
+      this.importRaw(await response.text(), type)
+    }
   },
 
   getColor: function () {

umap/views.py

@@ -335,7 +335,6 @@ showcase = MapsShowCase.as_view()
 
 def validate_url(request):
     assert request.method == "GET"
-    assert is_ajax(request)
     url = request.GET.get("url")
     assert url
     try: