Diffusion extension-CentralAuth 94b8bbb171b6

SUL3: Support signup/account creation action in SUL3 mode
94b8bbb171b6
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Description

SUL3: Support signup/account creation action in SUL3 mode

In this patch, when a user clicks on the "Create Account" link on
the local wiki, CentralAuth should detect if we're in SUL3 mode and
redirect the action initiator to a shared domain for them to create
their user account there (via a URL query parameter 'sul3-action').

This flows is captured in the following matter:
-> visit: local wiki
-> click: local wiki CreateAccount
-> redirect: local wiki UserLogin (param 'sul3-action')
-> redirect: shared domain CreateAccount
-> action: fill CreateAccount form on shared domain
-> redirect: local wiki UserLogin
-> action: user is logged in (on success)

An 'sul3-action' query parameter is used to direct what the primary
authentication provider should do on the shared domain. When it sees
the 'signup' action, it triggers account creation instead of the default
login.

NOTE: SharedDomainUtils has abstracted a method for treating various actions so that in the future, if we have other actions different from 'signup' or 'login', we can generate a URL there specific to the action on the shared domain and terat it.