Remove security-related legacy code
[Improvement] Replaced the Security Service with the static SecurityUtils to obtain a thread's current user and site

This change simplifies dealing with a thread's security information such as the current user and site and eliminates the need to pass around the security service that used to hold this information.

While the SecurityService api has been removed, the former SpringSecurityService has been renamed to SecurityServiceImpl and is now used to install implementations of java.security.SecurityManager as the System's security manager.

This change requires adapted (simplified) versions of the spring security configuration. Basically, all mentions of the SecurityService need to be removed (see sample security.xml as part of this commit).

+Configuration

Conflicts:
	modules/weblounge-common-api/src/main/java/ch/entwine/weblounge/common/site/Site.java
[Improvement] Corrected the wording in the area of security by renaming "permission" to "action"

While actions define operations that can be performed (like read, write), permissions define actions on a certain subject, e. g. read permission for page /news.

With this definition in mind, the codebase has been refactored accordingly.
[Improvement] Introduced a no-argument constructor to create a system-wide guest user
[Improvement] Moved evaluation of security constraints to the security manager.

Finalized the implementation of allow/deny policies with support for the order of evaluation of the allow/deny rules. All of the checks have been moved to the SecurityManager and the central SecurityUtils class. As of now, only the PageRequestHandler is securing content.

Conflicts:
	modules/entwine-videolounge/pom.xml
[Bugfix] Explicitly ordering resource content so that unit tests always work
[Improvement] Implemented support for implication of authorities, e.g. for inherited roles
Implemented proper reading and serialization of security context
[Improvement] Implemented security checks in the regular request handlers and the page list tag

Conflicts:
	modules/weblounge-taglib/src/main/java/ch/entwine/weblounge/taglib/content/PageListTag.java
[Improvement] Adding explicit logic to mark a user as logged in

Conflicts:
	modules/weblounge-kernel/src/main/java/ch/entwine/weblounge/kernel/security/UserContextFilter.java
[Improvement] Implemented extraction of the user as a convenience for content repository endpoints
[Improvement] Implement first round of access restrictions in endpoints
MANVL-119 -- [Improvement] Prevented links to protected pages
Make use of new security/authentication infrastructure
Fix bug in user to XML serialization
Serialize owner (user) according to XML schema
Move permissions check to avoid NPE
Replace hard coded READ action by action parameter
Improve parameter checking
Fix another problem with the XML serialization of securable resources