Highlight sentences when mouse hovering logs.

On a sentence page, highlight linked sentences when mouse hovering a related
logs entry. This should help a bit to understand the logs, e.g. what sentence
got created when. It’s probably not very usable for very long logs because the
displayed sentences easily get out of screen, but it’s better than mouse
hovering links back and forth while remembering sentence ids.
#362 Added thick border-right for highlighted log entry
Fix indentation.
Make log highlight touchscreen-friendly.

Let the highlight activated when hovering out, and allow highlight with
simple click too. This way, not only it allows touchscreens to properly
use this feature, but it also allow to scroll up to the displayed sentence
if it’s out of screen.
Prevent sentence-link-less logs to highlight.

So that these records can’t turn off the current highlight.
#362 Distinguish hover from click, added scrollTo()
#362 Remove hovered class when hovering out
Merge pull request #362 from Tatoeba/highlighted-logs

Highlight sentences when mouse hovering logs.