Write view adding identities to existing accounts

[jensens]

Add a view which enables users of Plone accounts or of other Authomatic accounts to connect a second, 3rd, ... identity to the account.

Place a link to the view in user preferences.

[jensens]

implemented: authomatic-handler view does login if anonymous or identity adding if authenticated.

open: csrf protection make adding stop working, even if safeWrite wrapped.

view need a link/action somewhere (probably as tab on personal-preferences).

[jensens]

fixed csrf protection issue with bd0ba69