Boost: Make sure multisites only check concatenation 404 status/GC once a day

[donnchawp]

A multisite install of WordPress may have any number of sites, each with their own copy of Boost. If they are all using the Concatenate modules, the plugin will check if wp-content can be used for storing those files once a day, but each site will perform this check. They will also schedule garbage collection for once a day.

The plugin needs to store a timestamp recording the last time the 404 tester was run so that it really only runs once a day on the entire network.
It also needs to do the same for garbage collection so that only runs once a day on the entire network.

The "on toggle" check, when either module is enabled from an all-off status, should only be performed by a local user with network admin rights to the website. There's no need for ordinary admin users who don't have write access to the server to perform these checks.

[github-actions]

OpenAI suggested the following labels for this issue:

• [Feature Group] Content Management: The issue pertains to managing the settings and operations of multiple sites within a multisite WordPress installation.
• [Feature] Concat: The issue discusses the Concatenate modules, emphasizing the need for a more efficient checking mechanism.
• [Feature] Performance History: This relates to the performance aspect of the Boost plugin, as it involves timing the 404 checks to optimize resource usage.
• [Boost Feature] Page Cache: The problem of checking 404 statuses once a day ties into caching mechanisms to improve site performance.