Advanced redirection after user/login

*Title: Advanced redirection after user/login.

*Incentive:

Currently, the redirect after login is set to the site root (or 
hardcoded from the login template). This is not very useful for sites 
with advanced permissions where some users can access parts of the site 
and others can access only other parts of the site. What we need is a 
system that allows the administrators to set up where users are 
redirected after login.

*Documentation:

Redirection URI can now be specified either per-user or per-group.

All you need is to add an attribute of type 'Text line' to class 'User'
and/or 'User group' and specify identifier of this attribute in an ini setting.
The attribute will then be used to tell where to redirect the user after login.

*New settings:

Two new INI settings have been added to site.ini:

[UserSettings]
LoginRedirectionUriAttribute[user]=<attribute_identifier>
LoginRedirectionUriAttribute[group]=<attribute_identifier>

The first one specifies which attribute of class 'User' contains
redirection URI for the user.

The second one specifies which attribute of class 'User group' contains
redirection URI for members the group.

*Examples:

To specify a "home page" for user Fred, you should perform the following steps:
1. Add a 'Text line' attribute to class 'User', name it 'Redirection URI'.
   The new attribute gets identifier 'redirection_uri'.
2. Edit your site.ini.append:
   [UserSettings]
   LoginRedirectionUriAttribute[user]=redirection_uri
3. Edit user Fred, put "/media" (or any other URI) to the 'Redirection URI' field.

That's it.
Since now, user Fred will be always redirected to the Media folder after login.

You can specify redirection URI for group "Guest accounts" in a similar way:
1. Add a 'Text line' attribute to class 'User group', name it 'Home page'.
   The new attribute gets identifier 'home_page'.
2. Edit your site.ini.append:
   [UserSettings]
   LoginRedirectionUriAttribute[group]=home_page
3. Edit group "Guest accounts", put "/news" (or any other URI) to the 'Home page' field.

*Implementation details:

If a user is member of several groups (= is a child of several
'User group' nodes), "main" group (= main parent node) is
preferred.

It is not mandatory for the classes to be 'User' or 'User group'.
The first class should just contain an attribute of type 'User account'.
The second class is just a class that is able to have children.

If the redirection URI has already been specified (e.g. via 'LastAccessesURI' session variable),
the advanced redirection feature gets disabled.
That means that if a user starts browser and goes to our.site.com/news,
he/she will _not_ be redirected to his/her "home page".