Refactor backend JS to remove jQuery dependency

[JakePT]

Description of the Change

This PR refactors the JavaScript for the Settings, Weighting, and Network Sites screens, as well as the JavaScript for handling admin notice dismissals, to remove jQuery as a dependency. It also fixes the occasional bug in the original scripts.

For the ElasticPress indexing switches in the Network Sites screen I took this as an opportunity to replaced the custom switch with the core WordPress toggle control for more consistency with the WordPress UI and better accessibility and handling of a busy state to avoid race conditions I encountered with rapid switching.

Closes #2074

Verification Process

For the Settings screen, verify that:

• Switching between tabs works.
• Switching between tabs appropriately updates the label and description of the host field.
• When switching between tabs you should be able to make changes to the host field without losing the value in the other tab.

For the Weighting screen, verify that:

• Adjusting the slider updates the number value in the field label.
• Unchecking the checkbox should disable the slider and update the number value to 0.
• Checking the checkbox should re-enable the slider and restore the number value to reflect the slider position.

For the Network Sites screen, verify that:

• The ElasticPress Indexing column has a toggle control in each row.
• Switching the toggle on and off should update the label to read "On" or "Off".
• After switching the toggle the toggle should be disabled while the AJAX request is running.
• After changing the toggle the new value should be reflected when refreshing the page.

For admin notices, verify that:

• When a notice is dismissed it should not reappear when refreshing or navigating to another page.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

• Changed: Refactored remaining admin scripts to remove jQuery as a dependency.

Credits

Props @JakePT

[JakePT]

@felipeelia With the failing tests I fixed one with a small tweak to to the test to reflect the change in markup, but the other tests that are still failing don't appear to be related. Let me know if that's wrong.

[felipeelia]

@JakePT Actually the input.click() was preventing the site from being indexed. I've pushed a new commit and if tests pass we can sync on the best time to merge this one. Thanks!