Use hash to identify the approved proxy admin function rather than storing the whole bytecode

[voltrevo]

What is this PR doing?

Uses hashes for these fields:

-    bytes public approvedProxyAdminFunction;
-    bytes pendingPAFunction;
+    bytes32 public approvedProxyAdminFunctionHash;
+    bytes32 pendingPAFunctionHash;

This saves gas. Then when setting the proxy admin function we can simply check the hash:

-        bool matchesApproved = (encodedFunction.length == approvedFunction.length);
-        for (uint i=0; matchesApproved && i<approvedFunction.length; i++) {
-            matchesApproved = (encodedFunction[i] == approvedFunction[i]);
-        }
+        bool matchesApproved = keccak256(encodedFunction) == approvedFunction;

How can these changes be manually tested?

Run the tests, in particular:
should register with new verification gateway

Does this PR resolve or contribute to any issues?

Resolves #118.

Checklist

• I have manually tested these changes
• Post a link to the PR in the group chat

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