Addons-server: Update Firefox logo in old front-end header

[jvillalobos]

Brought up in https://github.com/mozilla/addons/issues/712.

We expose old front-end pages in some cases, particularly the public statistics dashboard for some add-ons (example). We haven't planned when to port them to the new style, so it would be good to at least update the Firefox logo in the header.

[championshuttler]

Hi @jvillalobos , I would like to work on it

[jvillalobos]

Great. If you need any guidance, maybe @eviljeff can help?

[eviljeff]

for the header used on the statistic page you can just update the image in static/img/icons/

Looks like we have 6 other "firefox" images of various sizes in static/img/ though (excluding shield studies), which could also be updated - I'm not sure where they are used exactly, or if they are used at all still.

[ioanarusiczki]

I verified this on AMO dev with FF60(Win10). I also checked other old pages, new logo seems to be present everywhere.