[UX] Make form submit buttons "sticky".

This was brought up in #541 by @jenlampton:

We have problems with the location of the submit button in views, too. It's the only UI in core that puts it at the top right, and if your live preview is too wide, that submit button scrolls right off the page (scrolls to the right - where no one can find it). ...Here's an interesting idea, you know how we have a fancy new sticky-footer in the Bartik theme? Why don't we create a similar fancy new sticky-footer for forms where we can stash those submit buttons? The buttons would always be at the bottom left of the viewport, but we can let the page scroll behind.

This could be a handy UI pattern for any form that gets too long.

I recall bringing this issue up in d.org at some point, but I cannot remember where exactly or if it was for core or a contrib module like Module Filter.

Anyways, there is Sticky Edit Actions that does this in a really awesome way:

I think it uses Waypoints' Sticky Elements

~PR by @laryn: https://github.com/backdrop/backdrop/pull/2150~ ~~PR by @wesruv: https://github.com/backdrop/backdrop/pull/2237~~ PR by @jenlampton: https://github.com/backdrop/backdrop/pull/2292