Optimize stats, clean up code, respect settings

[yscik]

Fixes #2782 #2784

Stacked on #2776

Changes Proposed in this Pull Request

• Remove unused stats settings, only leave "Enable job statistics". Move it to the General settings tab since it's only one setting for now. Update labels.
• Make sure the setting is respected & stats are not logged or displayed when it's off
• Rename stats to make DB keys a bit shorter (job_listing_view -> job_view)
• Add constants, cleanup code
• Various tweaks for the frontend framework:
  • Organize and rename a few things for better clarity
  • Make stats triggered by a custom event/action (only page-load now) a dedicated stat type with it's own setup (action)
• And to observing search impressions:
  • Use a single IntersectionObserver to monitor the listing elements. Also use this for the initially visible listings, it'll trigger even when they are already in view
  • Clean up observer when a listing element is removed (this happens when search filters/keywords are changed)
  • Send the log request right away when 10 events are in the queue
  • Fix some timing issues causing nodes to be skipped sometimes
  • Move the job listing selectors to the PHP stat config, so theme authors can customize them

Testing Instructions

• In Job Manager settings, turn on the Job Statistics option
• Check that stats are logged correctly when viewing a job page ( job_listing, job_listing_unique, job_apply_click)
• Check that stats are logged when viewing the job list. Test with both ajax search and static: [jobs] and [jobs show_filters=false]
• Check that the above logging is only done when the settings is turned on
• Check that the Views column in Job listings admin and the job dashboard, and the Job statistics overlay is only shown when the settings is turned on

Screenshot / Video

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[thedebian]

Worked well, it needs a rebase though.
I had issues unrelated to this PR, reported on Slack.