Don’t try to update non-existent summary

Fixes the misleading error:

  TypeError: view() got an unexpected keyword argument 'dir_path'

Which was the result of making an AJAX request with empty URL that gets
interpreted as index page view() which is not prepared for AJAX requests
(and shouldn’t).
Remove non-existent logging bit that sneaked in

Those are related to logging changes in Evernote’s fork that we did not
pick, I failed to remove those while merging which broke submitting
translation! If we decided to pick the new logging code, this commit
should be reverted.
cache: Cache the top stats for a much longer time

Previous behaviour had us cache for x*POOTLE_CACHE_MIDDLEWARE_SECONDS
which is 10-15 minutes by default. That is ridiculously quick for
something that is very expensive to compute and doesn't change that
much.
By default we now cache for a whole day and this can be changed in the
new setting TOP_STATS_CACHE_TIMEOUT.
settings: Document POOTLE_TOP_STATS_CACHE_TIMEOUT