Correct over large user avatar in twentyfifteen at medium breakpoint changeover

[hnla]

Currently there's a conflict with the twentyfifteen user avatar rules for fluid images and cover header rules setting 100% wide #item-header-avatar.

Commit will remove the img.avatar % width to correct this issue in the twentyfifteen styles.

[hnla]

In 10292:

Twentyfifteen companion style correction.
Commit addresses an issue where the user avatar will be too wide just before small breakpoint cuts in.

See #6676