feat: use browser.contentScripts API in Firefox

This change enables Firefox users to disable creation of window.ipfs,
solving fingerprinting issue raised in:
#451

The key difference between tabs.executeScript and contentScripts API
is that the latter provides guarantee to execute before anything else.

Chrome does not provide contentScripts API and we need to statically
load content_script via  manifest.

More info on the underlying issue:
#368
#362 (comment)

add-on/manifest.firefox.json

@@ -18,6 +18,7 @@
     "default_title": "__MSG_pageAction_titleNonIpfs__",
     "default_popup": "dist/popup/page-action/index.html"
   },
+  "content_scripts": null,
   "protocol_handlers": [
     {
       "protocol": "web+dweb",

add-on/src/lib/ipfs-companion.js

@@ -30,6 +30,7 @@ module.exports = async function init () {
   var contextMenus
   var apiStatusUpdateInterval
   var ipfsProxy
+  var ipfsProxyContentScript
   const idleInSecs = 5 * 60
   const browserActionPortName = 'browser-action-port'
 
@@ -60,6 +61,7 @@ module.exports = async function init () {
     })
     modifyRequest = createRequestModifier(getState, dnsLink, ipfsPathValidator, runtime)
     ipfsProxy = createIpfsProxy(() => ipfs, getState)
+    ipfsProxyContentScript = await registerIpfsProxyContentScript()
     registerListeners()
     await setApiStatusUpdateInterval(options.ipfsApiPollMs)
     await storeMissingOptions(
@@ -95,6 +97,33 @@ module.exports = async function init () {
     }
   }
 
+  // Register Content Script responsible for loading window.ipfs (ipfsProxy)
+  //
+  // The key difference between tabs.executeScript and contentScripts API
+  // is the latter provides guarantee to execute before anything else.
+  // https://github.com/ipfs-shipyard/ipfs-companion/issues/451#issuecomment-382669093
+  async function registerIpfsProxyContentScript (previousHandle) {
+    previousHandle = previousHandle || ipfsProxyContentScript
+    if (previousHandle) {
+      previousHandle.unregister()
+    }
+    if (!state.ipfsProxy || !browser.contentScripts) {
+      // no-op if window.ipfs is disabled in Preferences
+      // or if runtime has no contentScript API
+      // (Chrome loads content script via manifest)
+      return
+    }
+    const newHandle = await browser.contentScripts.register({
+      matches: ['<all_urls>'],
+      js: [
+        {file: '/dist/contentScripts/ipfs-proxy/content.js'}
+      ],
+      allFrames: true,
+      runAt: 'document_start'
+    })
+    return newHandle
+  }
+
   // HTTP Request Hooks
   // ===================================================================
 
@@ -544,13 +573,16 @@ module.exports = async function init () {
         case 'useCustomGateway':
           state.redirect = change.newValue
           break
+        case 'ipfsProxy':
+          state[key] = change.newValue
+          ipfsProxyContentScript = await registerIpfsProxyContentScript()
+          break
         case 'linkify':
         case 'catchUnhandledProtocols':
         case 'displayNotifications':
         case 'automaticMode':
         case 'dnslink':
         case 'preloadAtPublicGateway':
-        case 'ipfsProxy':
           state[key] = change.newValue
           break
       }
@@ -616,6 +648,10 @@ module.exports = async function init () {
       contextMenus = null
       ipfsProxy.destroy()
       ipfsProxy = null
+      if (ipfsProxyContentScript) {
+        ipfsProxyContentScript.unregister()
+        ipfsProxyContentScript = null
+      }
       await destroyIpfsClient()
     }
   }