Add HTML 5 elements for number and range

This PR for https://github.com/backdrop/backdrop-issues/issues/200 contains 5 commits corresponding to 5 d.o issues about these elements.  Would love to get them all in, in one fell swoop.

There is one still unresolved issue on d.o about updating the documentation (or adding additional validation?) specifically for HTML5 number elements.   I have created a matching issue for Backdrop, here:
https://github.com/backdrop/backdrop-issues/issues/241