Added ItemDiscover, ItemCancel, and ScmTag permissions to the Permissions enum

[justinharringa]

These additional permissions should be useful for other standard permissions.

[jenkinsadmin]

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[daspilker]

Your pull request contains commits which do not belong to the pull request. Can you rebase your commits to the current master?

[justinharringa]

I have rebased my commits to the current master. I think this is ready to go now (still learning git). Thanks @daspilker!

[daspilker]

OK, great. Can you update the Job Reference and the Release Notes on the wiki?

[justinharringa]

Yep, I've added the information to both and did a little linking on the Job Reference page.

[daspilker]

OK, good so far. Can you update the Security section on the Job Reference page? It seems a little bit outdated. And can you replace the incomplete enum code block by an example or two?

[justinharringa]

I forgot to mention that I had updated the security section before (did some linking to the enum). But, I've now removed the enum duplication and added a few examples.