The institution logo feature didn't allow institutional admins to
revert to the standard logo, so once they'd uploaded a logo, they
couldn't go back.

This adds a delete logo checkbox on the institution settings page.

Change-Id: I16f0ecc1c0b5629662d2c463778c98799914ec28
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>