Add footnotes extension to showdown

refs 1318
- based on Markdown Extra https://michelf.ca/projects/php-markdown/extra/
- allows [^n] for automatic numbering based on sequence
Cleans up HTML data attributes on body in default.hbs

closes #4485

- removes data attributes used on body in default.hbs
- introduces new way to generate configuration through meta tags
- config initializer consumes configurations from the meta tags using parser
- moves blog_title helper/value to be a property in a configuration api
Use non-deprecated moment.substract()

Closes #4545
- Go with subtract(value, string) instead of subtract(string, value)
Merge pull request #4548 from felixrieseberg/iss4545

Use non-deprecated moment.substract()
Generate sitemap files

Closes #623

- Add basic init and eventing scaffold
- Add sitemap-index.xml generation
- Broke out generators to individual files, added request handler
- Add page, author and tag xml files; add index mapping
- Add SiteMapManager unit tests
- Add Generators tests
- Cache invalidation headers for sitemap-*.xml
- Redirect sitemap.xml to index and rename to sitemap-index
- Handle page convert and publish/draft changes
- Add very basic functional test for route existence
- Add cache headers to sitemap routes
Prevent silent failures and recover after error.

Refs #4543
- Alway return model out of save handler so sub-classes which
  call this._super() on save have a model on success and failure.
- Always return resolved promise out of post slug generator.
- Test the errors object in the error notification helper to avoid
  throwing an unhandled exception and silently failing.
- Consider a post model "dirty" if it is in an error state.
Do not display "auth failed" modal on clean posts.

Refs #4543
- Check EditorController.isDirty before displaying the
  authorization failed modal dialog.  This prevents the modal
  appearing immediately upon entering the editor in cases where
  auth has failed prior to opening the editor.
Update getting started link

No Issue

Fixed link anchor address that was changed
Merge pull request #4553 from jgable/fixSetupLink

Update getting started link
Merge pull request #4348 from jgable/sitemaps

Generate sitemap.xml
Allow notifications to sit over the nav bar

Closes #4379

- Adjusts the `.viewport` `z-index` propetty to allow notificatons to sit over the nav bar

Looks like a weird fix, but inheritence of z-index was the issue here.
Merge pull request #4547 from jaswilli/post-saving

Prevent silent failures and recover after error.
Merge pull request #4531 from Gargol/issue-4485

Cleans up HTML data attributes on body in default.hbs
Merge pull request #4551 from jaswilli/editor-401-modal

Do not display "auth failed" modal on clean posts.
Introduce sitemap XSL template

Based on Yoast's All in One SEO plugin for WordPress implementation
Merge pull request #4559 from JohnONolan/sitemap-xsl

Introduce sitemap XSL template
URL safe base64 encoding

closes #3872
- updated base64 escaping to respect + and \
- updated base64 escaping to remove = during transport
- updated tests
Cleanup layout Sass

Continues on from #4441

- Reformats the layout Sass files (comments, white space)
- DRY up small chunks of CSS to make it more comprehensible
Merge pull request #4554 from sebgie/issue#3872

URL safe base64 encoding
Move ugly debug to Settings  Labs

 closes #4534

- Adds new Labs route
- Wires route in settings page
- Move and rename debug templates and logic to labs
- Redirect /debug to settings/labs