improve styling of diff page

first steps towards:
https://mingle.corp.wikimedia.org/attachments/eb06b1db64417c279a7b481bb0edb7e5/215/Watchlist_UXSpec_Beta_Dec12th-06.png

Note:
Ignored following:
* tricky to separate edit count from message without html and not sure how well this will localise
* information about user not currently available
* position fixed styling will need to be reviewed
* no asset for + or - currently so ignored next to bytes added/removed

Change-Id: Ica229d80ce6d320fa39384e3066be1339a9743c5
Merge "improve styling of diff page"