- Fill Milestone and Assignee field when available in webhook payload

* Do not show the "Sign up for free" button in issue tracker

The "Sign in to comment" link is good enough and will correctly
show or not show the "Sign Up" button link for those not having
an account already.

Fixes #3407 (link to nowhere when registration is disabled)

* Move html from translation to template

* Remove extra space in `{{ .SignInLink }}`.

- add border-radius to filter list items
- use color as background of label for issue/pull view page

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