teams: membership events

Adding some relevant events to the Team Memberships service, so when they happen other modules of the system (i.e. WikiModule) can listen on it and continue the process out of the main thread.

From the doc:

Events serve as a great way to decouple various aspects of your application, since a single event can have multiple listeners that do not depend on each other.

Eventually, we could introduce more events throughout the code and detach some modules, so they execute in the background.

@OnEvent(TeamMembershipEvents.ON_TEAM_MEMBER_ADDED)
@OnEvent(TeamMembershipEvents.ON_TEAM_MEMBER_REMOVED)