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.