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.

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