Serve logo footer from assets

Summary:
It seems the footer has been adapted from the tools.nasqueron.org's one.

This logo is available as a static resource in assets. subdomain.

That's a more suitable source, as tools. is served by a back-end server,
and as such can be unreacheable for maintenance.

Test Plan:
Hashes of the two logos match.

Hash from tools

$ wget https://tools.nasqueron.org/images/Nasqueron.png
$ identify Nasqueron.png
Nasqueron.png PNG 133x131 133x131+0+0 8-bit sRGB 1516B 0.000u 0:00.007
$ optipng -o7 Nasqueron.png
** Processing: Nasqueron.png
...
$ md5 Nasqueron.png
MD5 (Nasqueron.png) = af5efed202464d635e63135e98b2ab23

Hash from assets

$ cd /home/dereckson/dev/nasqueron/assets
$ md5 logos/logo-main-133px.png
MD5 (logos/logo-main-133px.png) = af5efed202464d635e63135e98b2ab23

Reviewers: dereckson, ledesillusionniste

Reviewed By: dereckson, ledesillusionniste

Differential Revision: https://devcentral.nasqueron.org/D1968