Sitemaps correctly filter draft posts

fixes #4612

- adds missing line of code to remove draft posts
- adds tests to check drafts aren't added
Merge pull request #4614 from ErisDS/issue-4612

Sitemaps correctly filter draft posts
Update technical details version numvers

They were a bit dated. Nothing else.
Sort newest to oldest in sitemap files

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.
Added Highlight to Markdown Help
Merge pull request #4617 from dbalders/markdown

Added Highlight to Markdown Help
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

Sort newest to oldest in sitemap files
Tag Cover Images

Closes #4506

Works just like the Post Cover Images (and uses the same component)
Tiny changes to the component ensures that we can reuse it across Ghost