Remove uses of &mut JSRef

Since everything with JSRef happens with interior mutability, it doesn't
make any sense to use an &mut JSRef.
Fix padding on tables. This fixes the TOC on wikipedia pages.

Ref: #2554
Merge pull request #3416 from zwarich/remove-jsref-mut

Remove uses of &mut JSRef
Merge pull request #3421 from glennw/table-padding

Fix padding on tables. This fixes the TOC on wikipedia pages.