Revise setup instructions in README.md

* Set up local environment and clarified instructions in `README.md`
* Centralize on localhost:5000, since that's the default Foreman port
* Clearer GitHub and Stripe instructions

https://trello.com/c/TpFwhtlt/333-replicate-setup-locally-and-update-readme-me
Force UTF-8 encoding on file contents

`Base64.decode64` returns a string with encoding of `ASCII-8BIT`. Some linters
will try to write the file. If a file contains weird characters, an error will
get raised, like:

```
 Encoding::UndefinedConversionError: "\xE2" from ASCII-8BIT to UTF-8
```

Related error:
https://app.getsentry.com/app20126465herokucom/production/group/33706905/