ui-shared: make a char* parameter const

All cgit_xxx_link functions take const char* for the 'name' parameter,
except for cgit_commit_link, which takes a char* and subsequently
modifies the contents.  Avoiding the content changes, and making it
const char* will avoid the need to make copies of const char* strings
being passed to cgit_commit_link.

Signed-off-by: Jeff Smith <whydoubt@gmail.com>
Reviewed-by: John Keeping <john@keeping.me.uk>