Introduce new settings:
- ACCOUNT_HOME_URL, used by 'Site name' link on templates.
- ACCOUNT_DETAILS_URL, used on templates when user is connected.
'username' is registered as extra_data too for Clipper provider.
Template tag 'get_accounts_by_providers' is cleaner.
Remove useless account/settings view. Its content should be displayed
by some profile view.
SOCIALACCOUNT_ENABLED settings (of allauth package) doesn't work well.
Template tag 'is_socialaccount_enabled' provides a better check.
