Introduce html.h

All html-functions can be quite easily separated from the rest of cgit, so
lets do it; the only issue was html_filemode which uses some git-defined
macros so the function is moved into ui-shared.c::cgit_print_filemode().

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

ui-repolist.c

@@ -6,9 +6,10 @@
  *   (see COPYING for full license text)
  */
 
-#include "cgit.h"
 #include <time.h>
 
+#include "cgit.h"
+#include "html.h"
 
 time_t read_agefile(char *path)
 {