fix(notification): extract notification summary from $params

Fixes #6885
docs(notifications): Adds docs for  'object', 'action' and 'summary' params used by notify_user()
Merge pull request #7066 from juho-jaakkola/notification-summary

Extract summary from $params in notify_user()
feature(notifications): site_notification about an annotation can now have an URL

Fixes #7055
feature(likes): improves compatibility with notification plugins

This allows for example site_notifications about new likes to include a link to the liked entity.

Refs #7055
Merge pull request #7064 from juho-jaakkola/site_notifications

Improves notifications about new annotations