Prevent plugin pages access when upgrade pending

If a plugin's code has been upgraded, but the schema is still on the old
version, the plugin is effectively disabled (i.e. its entry points are
not displayed). However, it was still possible to access individual
pages directly, which could lead to errors.

This commit adds an extra check in plugin.php which triggers an error
when the plugin needs to be upgraded.

Fixes #16706
Fix group actions not sending notifications

Added missing email_generic() calls

Fixes #6343, #13659