REST: Make 'User.first_name', 'last_name' editable

REST: Make 'User.first_name', 'last_name' editable

In an ideal world, everything that can be done with the UI should also
be doable from the command line. The first and last names of the User
are low-value fields to allow customization of, but they are easily
implmented and move us towards the above goal.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Andy Doan <andy.doan@linaro.org>