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

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

Merges repo/sidebar.tmpl with repo/header.tmpl and makes every repo page use
middleware.RepoRef() necessary to display information on this menu.

This is more inline with way GitHub looks like and feels much more natural and
in style with rest of the interface.