text: Title: [feature] When viewing a diff with multiple changes since last view, default to the last viewed date
  Body: **Version and OS**
  v0.45.5
  
  **Is your feature request related to a problem? Please describe.**
  When I see a bolded watch in the watch overview, I click it and it autoscrolls to the diff text. The issue is it only includes the diff between the latest two copies of the watch, and multiple changes may have occurred since my last view, since the diff date selected is scrolled out of frame, it may not be obvious that other changes are hidden between separate diff dates.
  
  **Describe the solution you'd like**
  Since it seems last_viewed is stored in the database, it should be possible to select the left side comparison to be the newest history item since the last_view date and diff it against the most recent version.
  
  **Describe the use-case and give concrete real-world examples**
  Page A is set to be checked every hour and it does change every hour. 12 hours go by since the user last viewed the diff, and the default diff link only shows changes from the last hour, the user has to scroll up to select the last date.
  