Add gerrit-theme.html and also add footer links

It seems that when using gerrit-theme.html it overrides
wikimedia-polygerrit-style.html so we doin't get the logo or name.

A future patch will remove wikimedia-polygerrit-style.html
so we can deploy this as is without breaking anything.

Bug: T196835
Change-Id: Ib2061118279b606d12ea58dac72528aea106d622