Add pretty identifier

`Identity.identifier`, while being the **uid** in the identity provider it is also quite meaningless when displayed to users (i.e. For Github, it is a numeric id instead of the username). We should have a way to specify which data field of the identity from a provider is the one relevant to the user. Something along this line would be a first approach to tackle it.

``` python
@property
def pretty_identifier(self):
return self.data[provider_settings['pretty_identifier']] if provider_settings['pretty_identifier'] else self.identifier
```

``` python
IdentityProviders = {
'github': {
'type': 'oauth',
'title': 'GitHub',
'oauth': _github_oauth_config,
'identifier_field': 'id',
'pretty_identifier': 'username',
},
```

Start by locating the Identity.identifier implementation and the provider configuration represented by IdentityProviders. Trace how provider data is stored and accessed, then verify that a configured display field is used for the user-facing identifier while the existing identifier remains the fallback.

Written by the indexing model from the issue text.