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

* Added rel="noopener" to target="_blank" hrefs

* Replaced gogs.io/docs with docs.gitea.io

This commit improves templates readability, since all of them use consistent
indent with all template command blocks indented too.

1. Indents both HTML containers such as <div>, <p> and Go HTML template blocks
   such as {{if}} {{with}}

2. Cleans all trailing white-space

3. Adds trailing last line-break to each file