moodle-tool_mergeusers: added logs visualization.
    * Settings present two links on merging users, one for merging and another one for viewing logs.
    * Viewing logs lists a table with ALL the records, with helping links:
        * One link to see the full log of actions.
        * One link on every user field to bring the admin to the given user.
    * The plugin also detects whether the user is no longer present on database, showing that user with ID X was deleted.