Guest plugin is new plugin replacing hardcoded guest access logic in manual plugin.

This plugin only grants temporary guest access and guest role, the access may be password protected.

Benefits:

• separate guest section on course edit page eliminates confusion caused by a 3-state access selector
• password is not shared with self enrolment plugin any more
• it can be replaced by custom guest plugin
• the negative guest capability was completely removed
• improved performance in related accesslib code