An institutional admin modifying configurable theme colours doesn't
immediately see the new colours unless they are also changing the
theme dropdown.

This change ensures the user's theme is reset whenever the institution
settings form is submitted with a custom theme.

Change-Id: I3e39cf2de182fc6fa1cba10f4063412025f363ef
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>