ui-blame: add blame UI

Implement a page which provides the blame view of a specified file.

This feature is controlled by a new config variable, "enable-blame",
which is disabled by default.

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

cgit.h

@@ -228,6 +228,7 @@ struct cgit_config {
 	int enable_http_clone;
 	int enable_index_links;
 	int enable_index_owner;
+	int enable_blame;
 	int enable_commit_graph;
 	int enable_log_filecount;
 	int enable_log_linecount;