Fix - message sent to a list not associated with a campaign couldn't be shown in archive - to know which message to show even if the list is not at the campaign, we store test messages in table test_messages
It is now possible to have hierarchical merge tags (separated by "."). The merge tags are now case sensitive.
Mailtrain allows passing element "mt:entries-json" in the RSS item feed. If this is detected, it parses the json structure and makes it available through RSS_ENTRY_CUSTOM_TAGS. Then it can be used as [RSS_ENTRY_CUSTOM_TAGS.field_quote_text.rendered]
