Fix frontpage avatars (#13853)

The frontpage uses a rather strange method to obtain the commit's avatar
which I've overlooked earlier. I don't exactly understand how it works
but this change fixes the wrong default avatars by using the function
that was in previous use.

Also introduced a few constants for size an size increase factor.

Fixes: https://github.com/go-gitea/gitea/issues/13844

web_src/less/helpers.less

@@ -22,7 +22,7 @@
 }
 
 .full-screen-width { width: 100vw !important; }
-.full-screen-height { height: 100vw !important; }
+.full-screen-height { height: 100vh !important; }
 
 .rounded { border-radius: var(--border-radius) !important; }
 .rounded-top { border-radius: var(--border-radius) var(--border-radius) 0 0 !important; }