Changes

Fixes the message generation in the EventAlreadyJoinedException class
The message in the EventAlreadyJoinedException was intended to contain 
the user's login and the message's id, but it wasn't. This commit fixes 
this problem, changing the formatting method employed.

tsk14 Solves some inconsistencies in the event join action
The current implementation of the event joining action was inconsistent 
and caused some unexpected results arbitrarily. The main cause was the 
use of forms inside an repeat element in the JSF layer. This 
implementation has been changed and, instead of using a form, a link 
redirects the join request to a new joinEvent.xhtml page, that processes 
the request.

Adds functionalities for anonymous user check to the LoginUserController
This new functionalities make easier to check whether the current user 
is an anonymous user or not. Additionally, redirecting functions based 
on user anonymity were added to avoid illegal page access. This 
functions can be used as "preView" events, to avoid page rendering when 
the user can't access a certaing page.

Updates the version control style guide in the contributing guide
Now, if a commit is related to a specific task, the commit title must 
start with the task id.