More progress on ticket:1919

These messages now don't use JavaScript string concatenation but
variable interpolation:

 * browse.start_rjs.history_for_feature
 * browse.start_rjs.edited_by_user_at_timestamp
 * browse.start_rjs.object_list.selected.type.node
 * browse.start_rjs.object_list.selected.type.way
 * browse.start_rjs.object_list.history.type.node
 * browse.start_rjs.object_list.history.type.way

These previously extant messages have been deleted, merged, and given
more descriptive names:

 * browse.start_rjs.history_for
 * browse.start_rjs.edited_by
 * browse.start_rjs.at_timestamp