Use subtler highlighting for copied and moved code

Summary:
The highlighting is distracting according to Nick Shrock and others.
Real designer, Lee Byron, helped me with this.

It also gives us unagressive target for jumping to the source line in future.

Another feature I will probably implement is highlighting also the source of copies/moves.
I will use the right side of the left column for it.

Test Plan:
Hover copied notifier.
Hover coverage notifier.

I've also checked that this doesn't break our super-flaky old/new code JavaScript detector.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin, leebyron, schrockn

Differential Revision: https://secure.phabricator.com/D2443