Cache the audio_stats element.

Since the number of sentences with audio isn't updated often, we can set a
quite long caching time. But not too long, so that we don't have to wait that
long before saying 'hey look at these new audio files' to everybody.
Make mutable data of 'search_bar' as parameter.

So that we can conditionally cache it depending on these parameters.
Cache the search_bar element.

We only cache the 'empty' version of it, when no keywords are prefilled
and no from/to languages are selected. That is, everyone who did not search
anything so far will get a cached version. As long as a search is entered,
it stays as long as the session exists, so the performance boost isn't
that awesome.
Cache the google_analytics element.
Factorized and cached the 'join us' message block.
Cached the sentences_statistics element.

We must choose a short enough caching time value so that visitors can see
the numbers growing. But not too short neither so that it actually helps.
Cache the login box element.

The form can contain prefilled values, and we don't want these
to be cached. Otherwise, everybody will get the prefilled value
someone entered at the time the cache was refreshed. Prefill
actually happens when you go on the register page and send an
invalid form. So, always keep fields empty.
Comment the caching bevhaviour of search_bar.
Merge pull request #223 from Tatoeba/cache

Cached view elements