* Add WC Pay to allowed plugins
* Add WC Pay Note to Inbox
* Don't update WC Pay note content
Actioned notes are hidden from the Inbox it turns out, so there's
no need to update the content to reflect that the WC Pay plugin
has been installed.
* Add WC pay note only after April 14
Instead of only adding the Install Now button in April. Only
add the note so that the Install now button is always available
and generally, the note is more useful and actionable because
the plugin is actually available.
* Fix WC Pay note install action
* Check if WC Pay is active or installed
Not just if the plugin files are available. This addresses both the
case where the plugin may be active an in a different folder name,
and where the plugin may have been installed but is not active,
likely because it's been deactivated.
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