Support custom page flow during signup and signin steps

[hsluoyz]

For now, we support "prompt" items (aka OAuth items) in signup items table. But it's very limited:

1. Currently it only supports binding to additional third-party logins. But we want to support all kinds of inputs: text input, drop-down selection, pure HTML static block, etc.
2. Currently it only supports the current prompt page to show after signup or signin. We also want to support even "custom" pages like signup successful page.
3. All pages should be managed together and in the same way. Like "Signup items table", "Signin items table", they should be based on the same code instead of implemented separately. Ideally we will have "Prompt items table", "Forget password items table", "Signup successful items table" easily.
4. We may also need to use work flow as belows to define the transitions between those pages. So embedding a such work flow engine may be required.

[casbin-bot]

@tangyang9464 @JalinWang @imp2002