Add a favicon.

[eggpi]

In #177 (comment), @verdy-p proposed a really nice logo for the tool. We should look into serving that as a favicon.

[verdy-p]

For favicon, we need more simplifications (and transformations to the .ico format that can store several resolutions, absolutely needed for the smallest sizes about 16x16px).

I just made that to fit in a 45x45px square (even if the image is 512x512px), but it can still be improved. I just took care of correctly centering it and balance all margins and that it better fits the userbox on TWN. If you decide no something else, It's easy to change on TWN.

You may propose a small competition on Meta-Wiki or Wikimedia Commons to get some other proposals.
Note that there are other tools on ToolHub that are translatable on TWN and don't have a suitable logo, only some screenshots.

[bd808]

If the chosen logo is uploaded to Commons it can also be used on the https://toolhub.wikimedia.org/tools/citationhunt entry in Toolhub.