Discover Collection View: Call appearance methods on cells

[bjtitus]

📘 Part of: #2104

Modifies the Discover collection view cells to call viewWillAppear, viewDidAppear, viewWillDisappear, and viewDidDisappear (indirectly via beginAppearanceTransition and endAppearanceTransition) when the cell is shown or hidden according to UICollectionView delegate methods. This behavior reuses our existing code for each Discover view controller which loads content, triggers analytics events, and handles some timer additions (for the Featured Summary cell).

To test

1. Enable tracksLogging
2. Navigate to Discover
3. Verify that the discover_list_impression event is called when a cell is just about to be shown.
4. Ensure that cells are correctly loaded
5. Navigate to a category
6. Ensure that the discover_featured_page_changed event is not being logged while in a category
7. Close the category
8. Ensure that the discover_featured_page_changed event is logged again

Checklist

• I have considered if this change warrants user-facing release notes and have added them to CHANGELOG.md if necessary.
• I have considered adding unit tests for my changes.
• I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.