Fine tuning changes to click-to-subscribe code

Related feedback:
- https://github.com/uBlockOrigin/uBlock-issues/issues/763#issuecomment-691682195

Additionally, enable an existing subscription when
subscribing again to it.

src/js/messaging.js

@@ -1590,10 +1590,6 @@ const onMessage = function(request, sender, callback) {
     let response;
 
     switch ( request.what ) {
-    case 'applyFilterListSelection':
-        response = µb.applyFilterListSelection(request);
-        break;
-
     case 'inlinescriptFound':
         if ( µb.logger.enabled && pageStore !== null ) {
             const fctxt = µb.filteringContext.duplicate();
@@ -1624,7 +1620,7 @@ const onMessage = function(request, sender, callback) {
     case 'subscribeTo':
         const url = encodeURIComponent(request.location);
         const title = encodeURIComponent(request.title);
-        const hash = µb.availableFilterLists[request.location] !== undefined
+        const hash = µb.selectedFilterLists.indexOf(request.location) !== -1
             ? '#subscribed'
             : '';
         vAPI.tabs.open({

src/js/scriptlets/subscriber.js

@@ -55,18 +55,27 @@ const onMaybeSubscriptionLinkClicked = function(ev) {
         return;
     }
 
-    const href = target.href || '';
-    const matches = /^(?:abp|ubo):\/*subscribe\/*\?location=([^&]+).*title=([^&]+)/.exec(href);
-    if ( matches === null ) { return; }
-
+    const subscribeURL = new URL('about:blank');
+    try {
+        subscribeURL.href = target.href;
+        if (
+            /^(abp|ubo):$/.test(subscribeURL.protocol) === false &&
+            subscribeURL.hostname !== 'subscribe.adblockplus.org'
+        ) {
+            return;
+        }
+        const location = subscribeURL.searchParams.get('location') || '';
+        const title = subscribeURL.searchParams.get('title') || '';
+        if ( location === '' || title === '' ) { return; }
         vAPI.messaging.send('scriptlets', {
             what: 'subscribeTo',
-        location: decodeURIComponent(matches[1]),
-        title: decodeURIComponent(matches[2]),
+            location: decodeURIComponent(location),
+            title: decodeURIComponent(title),
         });
-
         ev.stopPropagation();
         ev.preventDefault();
+    } catch (_) {
+    }
 };
 
 document.addEventListener('click', onMaybeSubscriptionLinkClicked);

src/js/storage.js

@@ -326,7 +326,10 @@ self.addEventListener('hiddenSettingsChanged', ( ) => {
         const importedSet = new Set(this.listKeysFromCustomFilterLists(externalLists));
         const toImportSet = new Set(this.listKeysFromCustomFilterLists(details.toImport));
         for ( const urlKey of toImportSet ) {
-            if ( importedSet.has(urlKey) ) { continue; }
+            if ( importedSet.has(urlKey) ) {
+                selectedListKeySet.add(urlKey);
+                continue;
+            }
             const assetKey = assetKeyFromURL(urlKey);
             if ( assetKey === urlKey ) {
                 importedSet.add(urlKey);