[DANGEROUS] untrack all banners, replace with default banner (#9)

if this git repo is meant for all instances, it doesn't make sense that 4get.ca banners are stored here.
instead, I've added `banner/*` to a .gitignore file, so instances can clone/pull/push the repo without grabbing other people's banners or uploading their own.

making this change required deleting all the currently tracked banners from the repo. an unfortunate side-effect of this is that **if you have any of these tracked banners in your local version, pulling this commit WILL DELETE ALL OF THEM!!!!!!** pulling this commit properly while preserving tracked banners should be done by temporarily copying them to another directory, `git pull`ing, then copying them back.

I also added a default banner based on the default nginx page so new instances aren't bannerless.

Co-authored-by: cynic <kurisufag1@gmail.com>
Reviewed-on: lolcat/4get#9
Co-authored-by: cynic <admin@cynic.moe>
Co-committed-by: cynic <admin@cynic.moe>

.gitignore

@@ -0,0 +1,2 @@
+banner/*
+!banner/*default*