Replace hard-coded footer padding with new footer-padding variable (Fixes #1878)

[trentpeterson]

This change addresses the problem of having hard-coded padding in the footer. It is an improvement to make the footer more flexible.

Proposed solution

This small change adds a new $footer-padding variable. If the variable is not overridden the previously hard-coded default value is used.

Fixes #1878.

Tradeoffs

None - works the same as it did previously if the variable isn't overridden.

Testing Done

I tested with the variable enabled and disabled and it works as expected in my local project that utilizes node-sass for compilation. I also searched for any collisions with an existing $footer-padding variable and did not find any.

This is a tiny change. My only question would be about the consistency of padding customization. Some items have individual variables for vertical and horizontal padding, while others (like this one) have a single padding variable. I don't see an advantage of one over the other besides consistency, but maybe someone else does. An easy change regardless.

Thank you!

[jgthms]

Thanks!