Keep the (empty) data directory in the repo

We don't want the Jenkins jobs which run our various import tasks to
have to care about whether or not this directory exists already, so
let's ensure that it always exists after a deploy. We do this for the
log directory already (although that directory is overwritten with a
symlink on deploy).

This .gitignore change keeps the data directory, but ignores everything
in it except for the .gitkeep file, so we won't accidentally commit the
notmodules into this repo.

The presence of the .gitkeep file shouldn't cause any problems for the
imports - they always look for files inside one of the subdirectories,
not in the data directory directly.
Merge pull request #472 from alphagov/keep_data_directory

Keep the (empty) data directory in the repo
Tidy hits_helper_spec

* Remove duplication around TimePeriod stubbing
* Be explicit about a context that wasn't requiring setup
Make hits period strip use all hits as appropriate

* No longer reverts to summary when a time period is chosen
  on all hits
* further tidy spec to concentrate on conveying intent, not implementation
Merge pull request #473 from alphagov/fix-all-hits-summary-bug

Fix all hits summary bug
Remove "All time" link from Universal Analytics

* Was expensive
* This is just a removal of the link; appending period=all-time to a
  Universal Analytics URL will still, at present allow you to *try* to reach it.
Move knowledge of slugs away from views

* Add options to `View::Hits::TimePeriod.all` to allow exclusion of all-time period
* Also tidy up some related specs that were incorrectly nested under `describe '.all'`
Merge pull request #474 from alphagov/remove-universal-all-time-link

Remove "All time" link from Universal Analytics