Add debug tools

[acicovic]

Is your feature request related to a problem?

Besides some information about plugin options in our Site Health Info page, the plugin doesn't feature any debug tools or options.

Describe the solution you'd like

• Provide a Debug tab in the settings page with debugging options.
• Implement a Debug Bar extension.

We should think in more detail about what information or tools we want to add there.

[acicovic]

Suggestion by @mjangda:

It would be nice to have some detail about the secrets. For example, it's helpful to know if the secret is set at all. Or if it's the right value. A good way to do this would be to show a partial value (e.g. first 3 and last 3 characters).