Add mobile screen engagement calculation using the screen summary context (close #16)

[matus-tomlein]

Description

Issue #16

This makes use of the information in the screen_summary context entity to calculate mobile app engagement. The new schemas that this builds on are in review: https://github.com/snowplow/iglu-central/pull/1358

The following columns in the views table were updated/added:

• engaged_time_in_s
  • Calculated as the maximum foreground_sec.
• absolute_time_in_s
  • Calculated as the maximum foreground_sec + background_sec.
• horizontal_pixels_scrolled
  • Calculated as the maximum max_x_offset.
• vertical_pixels_scrolled
  • Calculated as the maximum max_y_offset.
• horizontal_percentage_scrolled
  • Calculated as the maximum max_x_offset / content_width * 100.
• vertical_percentage_scrolled
  • Calculated as the maximum max_y_offset / content_width * 100.
• last_list_item_index
  • New metric, not calculated on Web.
  • Calculated as the maximum last_item_index.
• list_items_count
  • New metric, not calculated on Web.
  • Calculated as the maximum items_count.
• list_items_percentage_scrolled
  • New metric, not calculated on Web.
  • Calculated as the (maximum last_item_index + 1) / items_count * 100.

New configurations:

• snowplow__enable_screen_summary_context
• snowplow__screen_summary_context

What type of PR is this?

• [x] 🍕 Feature

Checklist

• [x] 🎉 I have verified that these changes work locally
• [ ] 💣 Is your change a breaking change?

Added tests?

• [x] 👍 yes
• [ ] 🙅 no, because they aren't needed
• [ ] 🙋 no, because I need help

Added to documentation?

• [x] 📓 internal package docs (ymls, macros, readme, if applicable)
• [ ] 📕 I have raised a Snowplow documentation PR if applicable (Link here if required)
• [ ] 🙅 no documentation needed

[rlh1994]

Also can you add a changelog entry, and we'll need to update the docs to reflect this but that's not required to merge this PR.