Remove references to filenames.json

The use of this file has been deprecated in favor of other
Datasource methods.

Removed reference to filenames.json in OH, MD and ID datasources.

Removed the ``clear_filenames()``, ``filename_mappings()`` and
``update_mappings()`` methods from ``base.datasource.Datasource``.
These methods referenced ``filenames.json``.  As far as I could tell
with a quick grep, these weren't being called from anywhere.

Closes https://github.com/openelections/core/issues/112
Match special election flag in slug and filenames

That is,
20080617__md__special__general__us_house_of_representatives__4.html
and not
20080617__md__general__special__us_house_of_representatives__4.html

Address https://github.com/openelections/core/issues/113,
https://github.com/openelections/docs/issues/12
Remove old filenames.json files

They're no longer being used with the latest Datasource class
implementation.

Addresses https://github.com/openelections/core/issues/112
Use direct_links field from API

Use the direct_links field in the elections metadata API response
instead of the old direct_link field.

Addresses https://github.com/openelections/core/issues/114