Add setting to enable/disable extra links on index page

The summary/log/tree links displayed for each repository on the index
page lost some of their purpose when the header menu was added, so this
commit introduces the parameter 'enable-index-links' which must be set
to 1 to enable these links.

Suggested-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>