Add credential checks

This change adds authentication/credential checks to sensitive API
methods, depending on the access level granted to the authenticated
user. It also adds the logic to save the authenticated user to the audit
log entries.

Note: the protobuf definitions were updated in a backwards-incompatible
way (KillSwitchAuthorizedUser.AccessLevel enum). This can be done since
this product hasn't yet launched.

Fixed: twpowertools:46
Change-Id: I9bf888d6108f463369143610d4bd5b256035b68f
diff --git a/api_proto/kill_switch.pb.go b/api_proto/kill_switch.pb.go
index 4dd0884..f46160f 100644
--- a/api_proto/kill_switch.pb.go
+++ b/api_proto/kill_switch.pb.go
@@ -1,7 +1,7 @@
 // Code generated by protoc-gen-go. DO NOT EDIT.
 // versions:
 // 	protoc-gen-go v1.26.0
-// 	protoc        v3.17.1
+// 	protoc        v3.17.3
 // source: api_proto/kill_switch.proto
 
 package api_proto