Add protected branches actions to Audit events

Description

Someone changed the protected branches settings in one of our GitLab projects and we can't trace who did this (and why).

Proposal

Please add operations on protected branches, such as:

• protecting a branch
• unprotecting a branch
• change in permissions to a protected branch

..to Audit events.

Links / references

Please add this to https://gitlab.com/gitlab-org/gitlab-ee/issues/579.

added ~113221 label

ZD: https://gitlab.zendesk.com/agent/tickets/82289

added customer+ and removed customer labels

added audit events label

added backend label

added Platform label

added feature proposal label

mentioned in commit 32dae283