Check if downloadURL isset before rendering

[dgading]

fixes #4194

QA Steps

1. Build the demo site and install sample data. Visit a dataset /node page to make sure the template still adds the download urls to the page.
2. Create a new dataset. Fill in the required fields.
3. For the distribution section, add a new distribution, leaving Download URL section empty and adding an Access URL. This will make the frontend render this section and you will not see the error. (The access url does not render well since it didn't have an explicit variable set in the theme file.)

I've also added the variable metastore for templates to use. This is just the raw json object with no processing on it. This is the first step towards deprecating the dataset variables. The new metastore variable can be used in node--data templates by using something like metastore.title or metastore.keyword[0].

Warning: Undefined array key "downloadURL" in metastore_preprocess_node__data() (line 101 of /var/www/html/docroot/modules/contrib/dkan/modules/metastore/metastore.theme.inc)

[dafeder]

This accomplishes the goal of the ticket, which was to remove the error at the top. We should not as a known issue though that distributions do not render particularly coherently if there is an accessURL instead of a downloadURL.