Partially implement #770.
Add "Default Webhooks" page in site admin UI.
Persist to the existing webhooks table, but store with RepoID=0 and OrgID=0.
Upon repo creation, copy the set of default webhooks into the new repo.

* add more webhook support

* move hooks templates to standalone dir and add more webhooks ui

* fix tests

* update vendor checksum

* add more webhook support

* move hooks templates to standalone dir and add more webhooks ui

* fix tests

* update vendor checksum

* update vendor

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* load attributes when created release

* update comparsion doc

* implementation of discord webhook

* fix webhooks

* fix typo and unnecessary color values

* fix typo

* fix imports and revert changes to webhook_slack.go