remove ability to pull latest version of resources.txt from remote repo.

This is required as per Firefox extension reviewers. Mail exchange:

========

Reviewer:
> Do I read the code correctly that you are executing remote JS by
> downloading/updating from
> https://raw.githubusercontent.com/uBlockOrigin/uAssets/master/filters/resources.txt
> and injecting scripts in contentscripts.js?

Me:
> Yes, resources.txt contains scriptlets or other resources used to:
>
> - Minimize potential page breakage (e.g. google-analytics.com/ga.js);
> - Defuse anti-blockers (e.g. bab-defuser.js);
> - Defuse anti-blockers or minimize page breakage through redirection
> (e.g. 2x2-transparent.png)
>
> This is not a new feature -- this is also part of the legacy version,
> and I consider this is a major feature of uBO. Given how fast things can
> change out there, this allows me to quickly push fixes when a new issue
> is reported for a site without having to go through a full update of the
> extension.

Reviewer:
> I am aware that this is not a new feature. I am unclear why it has been
> allowed in the past, since it violates our policy about remote code
> execution. I assume it was missed due to the fairly complex codebase.
>
> I can approve this version so you are not blocked on the migration, but
> eventually, you cannot use functionality that executes remote code.
> Since we're moving to a more automated review process, you will be able
> to ship new versions without being blocked on a human review.

Me:
> Do I understand correctly that extensions such as TamperMonkey or
> ViolentMonkey won't be allowed on AMO?
>
> Those extensions are even more permissive than uBO given a user can
> import scripts from any source, while with uBO only scriptlets which are
> part of the project are allowed.

Reviewer:
> The key difference between add-ons like Tampermonkey and uBO is that in
> Tampermonkey, users are making an active and conscious decision to
> download and execute that specific code. In uBO, the user did not
> initiate that download/execution, nor are they even aware of it
> happening.

Me:
> So users of TamperMonkey -- tech-savvy or not -- can download & inject
> countless 3rd-party user scripts from countless authors, have them
> update on their own automatically at regular interval with no user
> intervention.
>
> On the other hand, it's not acceptable for me, the author of the
> extension, who users implicitly trusted when installing the extension,
> who is completely controlling and vouching for the content of
> "resources.txt", to have this one 1st-party resource file[1] to be
> updated at regular interval with no user intervention.
>
> So anyways, what is expected from me at this point? Do I need to remove
> scriptlet injection and resource redirection features? Do I need to
> remove only the updating part of resources.txt?
>
> [1] key to core features of uBO (counter anti-blockers + page breakage
> mitigations) and possibly an important factor in installing the
> extension.

========

Now about this commit: the purpose of the code change here is to
prevent "resources.txt" -- which is part of the package -- from being
updated -- this applies only to the Firefox webext[-hybrid] version
of uBO.

src/js/assets.js

@@ -985,6 +985,22 @@ var updateFirst = function() {
     updateNext();
 };
 
+// Firefox extension reviewers do not want uBO/webext to fetch its *own*
+// scriptlets/resources asset from the project's *own* repo (github.com).
+var noRemoteResources = false;
+(function() {
+    if ( 
+        typeof browser === 'object' &&
+        browser !== null &&
+        browser.runtime instanceof Object &&
+        typeof browser.runtime.getBrowserInfo === 'function'
+    ) {
+        browser.runtime.getBrowserInfo().then(function(info) {
+            noRemoteResources = info.vendor === 'Mozilla';
+        });
+    }
+})();
+
 var updateNext = function() {
     var assetDict, cacheDict;
 
@@ -1007,6 +1023,10 @@ var updateNext = function() {
             if ( cacheEntry && (cacheEntry.writeTime + assetEntry.updateAfter * 86400000) > now ) {
                 continue;
             }
+            // Update of user scripts/resources forbidden?
+            if ( assetKey === 'ublock-resources' && noRemoteResources === true ) {
+                continue;
+            }
             if (
                 fireNotification(
                     'before-asset-updated',