Move and reword Security section

[benfrancis]

As originally proposed in #73 (comment) and #78 this PR moves the Security section out of the WoT Core Data Model section into its own section within Core Profile.

I've also refined the wording of the normative statements and added references to the WoT Thing Description and WoT Security Security Best Practices documents.

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[mlagally]

I'm getting more and more concerned by MRs that contain textual and structural changes in a single MR.
They require significant time to review, since there's no useful diff.
Please refactor into a MR that only proposes the text changes.
If you also want to propose structural changes of the document, please create a separate MR.

[benfrancis]

@mlagally When I split the large PR into six separate PRs, the reason that I kept these changes together is that the one change builds on the other, which means that two separate PRs would cause a merge conflict.

I've now split this PR into two separate commits so you can see the diffs separately, but I'm not sure it actually makes much difference.