Fix events order in users/:id/events endpoint

Order of events in contributions API is currently being lost, though docs are saying:

> Get the contribution events for the specified user, sorted **from newest to oldest**.

Order becomes different after `.merge(ProjectsFinder.new.execute(current_user))` call, so I moved ordering below this line.

This MR also removes extra `.page(params[:page])` call in the method chain, since [`paginate(events)` already does it](https://gitlab.com/airat/gitlab-ce/blob/master/lib/api/helpers.rb#L112).

See merge request !7039
Signed-off-by: Rémy Coutable <remy@rymai.me>