Stop DDOSing ourselves
## What does this MR do?
Adds `hiddenInterval` to `SmartInterval` settings. This will be the interval used whilst the tab is inactive. If this setting is not set it will `cancel` as it would have done before.
Adds `immediateExecution` to `SmartInterval` settings. This boolean will dictate whether to execute the callback once before the first interval or not.
Uses `SmartInterval` with the new `hiddenInterval` settings to slow the polling for the MR widget on inactive tabs.
Fixes issue where `SmartInterval`s `visibilitychange` listeners are not getting called because jQuery doesn't support them.
## Are there points in the code the reviewer needs to double check?
## Why was this MR needed?
We're ddosing ourselves on the MR page.
## Screenshots (if relevant)
~30 seconds difference between the first 2 requests _(a couple requests had fired already so it's backed off already)_.
_-changed to different tab-_
~2 minutes difference between the middle 2 requests.
_-changed tab back to MR-_
~10 seconds difference between the last 2 requests.
## Does this MR meet the acceptance criteria?
- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
- [x] Added for this feature/bug
- [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 #24807
See merge request !7762
