Enhance the latest contributions page.

Display differently contributions that are not the latest one for a given
sentence, so that these can be easily ignored when glancing at the latest
contributions.
Logs visual rework

I didn't feel very comfortable having obsolete logs displayed with a stroke text. It gave the impression that the sentence was deleted.
I couldn't find a satisfying way to express that a log entry was obsolete based on the previous design so I sort of redesigned the whole thing.
Obsoletize latest contributions on homepage.
Some more visual changes for logs.

- Moved the date to the left side.
- Pastelized colors.
- Changed the padding and margins for clearer separations between
entries.
Members currently contributing in latest contributions annexe.
Merge pull request #477 from Tatoeba/latest-contribs

Enhance the latest contributions page.