Compatibility with WPBP 1.4+

[patrickposner]

We should revisit why the plugin isn't compatible with the latest version of WordPress Background Processing: https://github.com/deliciousbrains/wp-background-processing/releases/tag/1.4.0

There are some features we want to leverage in Simply Static as well, like:

• the new chain ID implementation
• we might have the ability to add a little overview screen on currently running tasks
• we should also incorporate the hotfix for MU sites: Run batches in multisite within context of site that created them deliciousbrains/wp-background-processing#128