moodle-tool_mergeusers - WIP: revisiting contribution code

    * Removed unnecessary translation strings. Replaced by Moodle core strings.
    * Translated strings to Catalan and Spanish.
    * Add a new settings option to enable merging only when transactions are allowed (but not used yet).
    * Moved the "Save selection" button at the bottom of the box for conformance among Moodle specifications.