Static file, URL, and entrypoint

The changesets introduce Flask builtin `url_for` callable to resolve URL. This eliminates hardcoded URL in email and HTML templates. Also, since `url_for` resolves everything for us, `users/static.py` is no longer needed.

As a bonus, a new entrypoint named `manage.py` is included to replace `runserver.py`.

```
usage: manage.py [-?] {shell,runserver} ...

positional arguments:
  {shell,runserver}
    shell            Runs a Python shell inside Flask application context.
    runserver        Runs the Flask development server i.e. app.run()

optional arguments:
  -?, --help         show this help message and exit
```

Example:

```
USERS_CONFIG=/path/to/config.py USERS_DEBUG=1 python manage.py runserver
```

It's worth to remember, neither `manage.py` nor `runserver.py` is suitable for production environment.