Resolve "Activity entries are not loaded fully to cover full window's height"

## What does this MR do?

Fixes a bug in which a browser window large enough to encompass the default number of items in an auto-loading endless-scroll list could not trigger the loading of additional items.

## Are there points in the code the reviewer needs to double check?

I refactored activities.js and pager.js into es6 syntax while I was investigating the issue.  No changes were made other than some es6 flourishes and eslint conformity, so nothing should be effected, but perhaps best to double check other pages which utilize these scripts.

## Screenshots (if relevant)

![endless-scroll-fixed](/uploads/b672b606826af745528c209ab6cfe95c/endless-scroll-fixed.gif)

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- Tests
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #24499

See merge request !7608