[mv3] Add managed setting to disable first-run page

Related discussion:
- https://github.com/uBlockOrigin/uBOL-issues/discussions/61

Example of usage (chromium/linux):

    {
      "3rdparty": {
        "extensions": {
          "ddkjiahejlhfcafbddmgiahcphecmpfh": {
            "disableFirstRunPage": true
          }
        }
      }
    }

platform/mv3/extension/js/background.js

@@ -31,6 +31,7 @@ import {
     runtime,
     localRead, localWrite,
     sessionRead, sessionWrite,
+    adminRead,
 } from './ext.js';
 
 import {
@@ -317,9 +318,12 @@ async function start() {
     );
 
     if ( firstRun ) {
+        const disableFirstRunPage = await adminRead('disableFirstRunPage');
+        if ( disableFirstRunPage !== true ) {
             runtime.openOptionsPage();
         }
     }
+}
 
 try {
     start();

platform/mv3/extension/managed_storage.json

@@ -6,6 +6,10 @@
       "title": "List of domains for which no filtering should occur",
       "type": "array",
       "items": { "type": "string" }
+    },
+    "disableFirstRunPage": {
+      "title": "Disable first run page",
+      "type": "boolean"
     }
   }
 }