feat: include models for pageview engagement

[SoryRawyer]

This includes models to support engagement dropoff analyses and page view completeness, i.e. how many learners viewed all pages in a section or subsection.

fact_navigation_dropoff is a model with two different aggregation levels: total views are counted for each section and subsection.

fact_navigation_completion counts the number of pages in each section and subsection and joins that to data about page visits. Aggregations can use the page_count column to compare against the number of unique pages a learner has visited. If the numbers are equal, then the learner has visited every page in the section.

Here is an example query of how fact_navigation_completeness can answer the question "which learners viewed all pages in a subsection?":

SELECT
    subsection_with_name,
    actor_id,
    page_count,
    countDistinct(block_id) AS pages_visited,
    page_count = pages_visited AS visited_all_pages
FROM reporting.fact_navigation_completion
GROUP BY subsection_with_name, actor_id, page_count
HAVING visited_all_pages = 1

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