Lock settings based on constants

[pfefferle]

This locks the Actor settings based on the ACTIVITYPUB_SINGLE_USER_MODE, ACTIVITYPUB_BLOG_MODE or ACTIVITYPUB_ACTOR_MODE constants.

Before:

After:

This is the first step to enable the settings on WordPress.com without confusing the user!

Proposed changes:

• It locks the settings to the Actor-Mode that is defined by a constant, if it is.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Go to '..'