Allow to control which of the optional fields from the registration page
are being displayed to the user. Added the configuration variable
`REGISTRATION_OPTIONAL_FIELDS` to control which ones. The default value
shows all the fields as currently; to not display some of the fields,
set the configuration variable with only the fields you want to display:
```
REGISTRATION_OPTIONAL_FIELDS = [
'level_of_education',
'gender',
'year_of_birth',
'mailing_address',
'goals'
]
```
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