conf: made the default webhook timeout duration longer. (#4423)

A 5 seconds timeout is too low and can cause random timeouts in certain setups.

conf/app.ini

@@ -214,7 +214,7 @@ TYPES = gogs, slack, discord
 ; Hook task queue length, increase if webhook shooting starts hanging
 QUEUE_LENGTH = 1000
 ; Deliver timeout in seconds
-DELIVER_TIMEOUT = 5
+DELIVER_TIMEOUT = 15
 ; Allow insecure certification
 SKIP_TLS_VERIFY = false
 ; Number of history information in each page