Multisite: Allow setting the site ID for multiple subsites simultaneously

Is your feature request related to a problem?

We got the following report from a user:

I didn't end up using the plugin because when activated for MultiSite setup, it still required setting the settings per blog and we would have to configure about 1000 blogs.

We do have some reports of similar cases. As things stand, managing Site IDs in a multisite setup with lots of subsites is a chore. An important information is that very often, the Site ID is identical for all subsites.

Describe the solution you'd like

A simple approach would be to add a setting or tool that would allow to copy a given site ID to all subsites.

A more granular approach would be to give the user the ability to add a given Site ID to specific subsites using an interface with checkboxes (including a "select all" checkbox). For example, the OneLogin SAML SSO plugin has a feature like this.

Start by reviewing the existing Site ID settings flow for WordPress multisite and the two approaches described in the issue: copying one ID to all subsites or selecting specific subsites. Done means administrators can apply a Site ID across the intended subsites without configuring each blog individually.