Added option to disable webhooks (#13176)

Added option to disable webhooks (#13176)

* Added option to disable web hooks

This mod introduces DISABLE_WEB_HOOKS parameter in [security] section
of app.ini (by default set to false). If set to true it disables web
hooks feature. Any existing undelivered web hook tasks will be cancelled.
Any existing web hook definitions will be left untouched in db but
its delivery tasks will be ignored.

Author-Change-Id: IB#1105130

* Webhook spelling fixed

Webhook spelling fixed.

Fixes: 07df6614dc84cdd2e9f39c57577fa1062bd70012
Related: https://github.com/go-gitea/gitea/pull/13176#pullrequestreview-510868421
Author-Change-Id: IB#1105174

* Parameter description fixed

Parameter description fixed.

Fixes: 07df6614dc84cdd2e9f39c57577fa1062bd70012
Related: https://github.com/go-gitea/gitea/pull/13176#pullrequestreview-514086107
Author-Change-Id: IB#1105174