Moved the code to generate relative dates to the client-side.

Since template fragment caching is used for edit units, it would always report
the same relative datetime. By moving these calculations to the client, we
always get an accurate relative datetime which is periodically updated.

Fixes bug 2706.
Several layout fixes.
Make last translator's gravatar more prominent.
Avoid animation glitches when toggling visibility.
Display comment's date and time.

This aligns with other parts of the timeline. Also refactored comment templates
to avoid repeating the same markup.
Provide proper separation for suggestions.
Fixed template indentation.
Unified markup.