Custom plugin for WP

See #101 (comment 34776):

@tin tried to upgrade WP from within the app from 5.8.1 to 5.8.2, which should not made possible in a containerized env. So we need to disable in-app upgrades.

New issue definition (TG):

Develop a custom plugin, shipped by default, that does the following:

• disable in-app upgrades of WP (probably by hiding the options and alerts)
• show Stackspin-related content boxes on Dashboard pages where we need to explain how the Stackspin WP is different from regular WP
• disable user de-activation of required default plugins (OpenID, Stackspin plugin)