Fix line splitting in `white-space: pre` flows

The newline position from one line was getting used repeatedly, causing extra
"phantom" newlines in the following lines.  Fixes #3413.
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 http://doc.servo.org/ to redirect to /servo/index.html again...

... rather than being the Rust documentation index.

The order of `cp` command determines which `target/doc/index.html` file
overwrites the other.
Merge pull request #3414 from mbrubeck/pre-line-break

Fix line splitting in `white-space: pre` flows. r=gw
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.
Fix img with display: block. Fixes text/image overflow on wikipedia.

Ref: #2554