Reset the last_changed_sites option when a new site is created

[dkotter]

Description of the Change

We currently have some caching around our internal sites list that should be updated anytime a new site is created. An issue was reported where if a site was created using WP-CLI, no connections would show until that site was manually saved in the network settings. This seems to point to a caching issue.

Looking through the code that fires when a new site is created via WP-CLI, there is a possibility that the current way we update our cache may not fire correctly, under certain circumstances.

This PR adds a new hook onto wp_insert_site that fires our same cache clearing function we currently use that is hooked to clean_site_cache. In theory, this second hook should always fire, while the first one may not fire in certain circumstances. For most circumstances though, both of these hooks will probably end up firing.

Alternate Designs

None

Benefits

The internal connections list should be more accurate now when a new site is created

Possible Drawbacks

For most setups, I think we'll end up with both hooks firing, which isn't a huge deal but we'll basically clear the cache twice.

Verification Process

Within an existing multisite, create a new site, go to that site and go to the Pull screen. Ensure connections show on that screen. Run this test both with manually creating a site and using WP-CLI.

Can also test from the perspective of an existing site. So create a new site, go to an existing site, go to the Pull screen and ensure the newly created site shows in the list.

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.

Applicable Issues

#639

[dinhtungdu]

LGTM 👍🏼