Add script to blend in other limn data repository

Some parts of this repository's dashboards, graphs, ... rely on files
that have been factored out to the geowiki repository by erosen.

For those components, it is not sufficient to make the required files
available to limn through a separate directory in limn's data
directory. Limn does not pick those up. Those files actually have to
be in the same subdirectory of limn's data directory as the checkout
of this data repository. That ruins erosen's effort to get only part
of the geowiki files into the geowiki repositories, and we're stuck
with a broken dashboard-data repo.

However, it seems the dashboard repository itself does not have code
to generate those files, as it's geowiki directory is outdated.

To be able to move forward, and arrive at a working data-repository,
we add script that allows to link in a known good checkout of the
geowiki-data repository. To do this, run

  ./blend_in_repository.sh --clean-links $ABSOLUTE_PATH_TO_GEOWIKI_DATA_CHECKOUT

Change-Id: I00e44782a7eba9cdf9d88cf5a32793084c68431e