Data model compatibility for pipelines without web page context enabled

@christophe,

The new data models are a huge help in getting the data modeling step off the group. Thank you!

I noticed that the data model relies on the web_page_context, however, if the users pipeline does not enable the web page context or enabled it after the pipeline was up and running the events that were recorded prior to the date the web page context was enabled will simply be omitted.

Do you have any recommendations for how to make the data model compatible with optional snowplow features, or in our case backwards compatible to also include events that do not have a page view id?