Make the event module really optional

It shouldn't be installed by default, only if the user wants to.
The application should run properly either with or without the module.

Now it is optional in the Gemfile and all references to MwebEvents and its
classes are protected to run only if the module was loaded. The initializer
now sets flags to tell the application if the module is loaded/enabled.

refs #1115
Improve how we check if the event module is enabled

Created a base module with a method to check if a module is enabled.
This method is automatically available as helpers for the views, and in
models and controllers it can be used by including/extending Mconf::Module.
Merge branch '1115-events-optional' into 1115-events

Conflicts:
	Gemfile.lock
	app/models/abilities.rb
	config/initializers/mweb_events.rb