Implement generic per-site module and variable monitoring api

It is a common task that a plugin monitors a set of variables and modules/themes, but currently it is not possible for the admins to ignore certain monitored module or variables (eg. SEO plugin monitors the presence of a bunch of modules that are not necessarily needed on a certain site).

Each site stores a list of monitored variables and modules and the plugin that monitors them, these should be ignorable, configured by the user. Configuration UI should be probably on the node form for starter.

This API should be able to store:
- the name of the plugin that wants to monitor given variables/modules
- the node id
- the name of the variable or module
- the type of the settings (=variable or module)
- ignore status

Each monitoring plugin implements a hook with a list of monitored variables and modules it monitors. Also define a description text for each one of them. Admin UI to configure the ignore status is provided by this new API module.