Add group actions to Audit events

Description

As part of the Audit events, we need to log important group actions. This is to be recorded when something happens at the group level:

• Group created
• Change group visibility
• Add member
• Remove member
• Add project to the group
• Remove project to the group

Additional information to record

• Who did the action
• Timestamp
• IP address

This information is available instance wide, in a new tab in the administration panel, under 'Audit events'.

Wireframes

References

Part of meta issue about logs: https://gitlab.com/gitlab-org/gitlab-ee/issues/579

mentioned in issue #579

mentioned in issue #1449

added SP2 customer labels

https://gitlab.zendesk.com/agent/tickets/70025

changed milestone to %9.4

changed milestone to %10.0

added Deliverable label

changed milestone to %10.1

changed milestone to %10.2

assigned to @jramsay

added backend label

added EE Premium label

removed EE Premium label

added EE Premium label

assigned to @fjsanpedro