Further simplify the Heroku cookie

Remove `CookieFixer`. Make set logic purely based on Identity's main cookie.
Register the extension in `Default` and `Account` as well.

Identity doesn't currently seem to be doing a good job of setting this
cookies back after its been manually removed by a user. The
`CookieFixer` was supposed to be doing this job, but it seems to have an
ordering problem when layered on with `HerokuCookie`. Simplify this
interaction by combining these both into a single extension.

Fixes heroku/help#159.