Update download extension icons in readme

[SecretX33]

Issue: #1160

As stated in the Issue, I have both the square and rounded versions of the Chrome icon, here in this PR I opted to use the squared version (to be as similar as possible to the current icon), but I personally think the round icon looks the best, so maybe we could switch to that.

Another thing to note in this PR is that the colors of the old and new Firefox download icon are slightly different (the new one has a darker blue), but that is easy to modify since it is a high-quality PNG picture, it is a matter of copying the color of the old icon and applying it on the new one using Paint/Photoshop.

And last but not least, I wasn't sure what the convention for icons is for FastForward. I wasn't sure if the icons should be somewhere in the project's folder (like in the docs folder), or if it should be hosted externally. Since the current icons being used are from user-images.githubusercontent.com , I assumed the project wants to keep its icons externalized, so I uploaded them to imgur.com (but no clue if this was the preferred way).

Here's the upload link with all three icons: https://imgur.com/a/2luHAqC

Fix(es):
There was no fix, this PR updates the Firefox and Chrome download icons in the project readme.

• I made sure there are no unnecessary changes in the code;
• Tested on Chromium (Includes Opera, Brave, Vivaldi, Edge, etc);
• Tested on Firefox.

[NotAProton]

using the https://github.com/FastForwardTeam/assets repo would be preferred instead of imgur but we'll have to edit it's readme

The respective authors of all files in this repository have, to the extent possible under law, waived all copyright and related or neighboring rights to the files.

to say that the files in the logo and logo_new are under CC0 and the Chrome, Edge and Firefox logo in the ext directory are the property of their respective owners.

[driedpampas]

FastForwardTeam/Assets#2

[github-actions]

Download the artifacts for this pull request:

• FastForward_chromium.zip
• FastForward_firefox.zip

[lostdusty]

Looks fine to me, not like imgur will die soon.