Make VC helper fields configurable

Adds KALAMAR_VC_HELPER_FIELDS environment variable and vc_helper_fields
configuration option to add or remove fields from the virtual corpus
builder at runtime.

Syntax: KALAMAR_VC_HELPER_FIELDS=+award:text,-docTitle
  +name:type  adds a field
  -name       removes a field

The result is always sorted alphabetically. Analogous to
KALAMAR_HINT_FOUNDRIES (aa6709c).

Required for DeLiKo@DNB

Change-Id: If9ad570133f78515aa6ed0ef87424c0843e76d9c
diff --git a/templates/layouts/main.html.ep b/templates/layouts/main.html.ep
index 749dec3..b3bd049 100644
--- a/templates/layouts/main.html.ep
+++ b/templates/layouts/main.html.ep
@@ -59,6 +59,7 @@
         % $api =~ s!/$!!;
         data-korap-url="<%== $api %>"
         data-hint-foundries="<%= join(',', @{stash('hint_foundries') // []}) %>"
+        data-vc-helper-fields="<%= stash('vc_helper_fields') // '' %>"
         itemscope
         itemtype="http://schema.org/<%= stash('schematype') || 'WebApplication' %>">