Display last translator gravatar next to the editing area.

Also adapted other gravatar's markup for consistency and removed unneeded extra
markup.

Fixes bug 2405.
Simplified code by using setdefault.
Removed dead code.
Use insertAfter for proper resulting markup.
Display the date of the latest edit below the edit box.

Now if the latest entry in the timeline refers to a change made only to the
target, it's not displayed again as part of the timeline, since the information
is already available as part of the current unit status.

Fixes bug 2702.
Some layout fixes.