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Closes #4611

Refactored generateXmlFromNodes to pull the urlElements itself from
sorted values in the lookup

Added some checks to existing unit tests to validate ordering.
Add Tag Meta View

Closes #4509, Closes #4615

- Adds meta title & meta description input fields
- Autosaves meta data
- Shows SEO preview (falling back to tag name & description if meta not supplied)
- Adds `type="button"` to delete button (which closes 4651)
Merge pull request #4565 from PaulAdamDavis/tag-meta

Tag Meta Screen
Merge pull request #4616 from jgable/siteMapOrder

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Merge branch 'master' of https://github.com/TryGhost/Ghost

Conflicts:
	core/client/controllers/settings/tags.js
	core/client/templates/settings/tags/settings-menu.hbs