ui-repolist: Allow sections to be collapsible

ui-repolist: Allow sections to be collapsible

The index page can be difficult to navigate for really large git
servers. This change allows a configuration like:

 section-collapse=people
 section-collapse=tests

And an index page would only display the "people" and "tests" section
headers entries (not their repos) with a hyperlink that can be used to
drill down into each section.

Additionally the boolean logic around displaying sections in
ui-repolist.c was simplified to eliminate an impossible condition.

Signed-off-by: Andy Doan <andy.doan@linaro.org>
Reviewed-by: John Keeping <john@keeping.me.uk>
Signed-off-by: John Keeping <john@keeping.me.uk>