Refactor auth form styles

Closes #4246

- Removed the input wrapper background colour
- Moved `:focus` styles to one element, so its not defined separate for each page
- Removed unnecessary MQ for body background colour (it changed nothing)
Added open graph tags for ghost head helper

issue #3900
- uses isPrivacyDisabled helper to see if useStructuredData has been disabled in config.js
- adds an array of promises to deal with asynchronous data
- resolves asynchronous data then adds open graph tags after canonical link
- featured image and tags are only added if present
- open graph tags only added on post and page
- adds unit test to check correct data is returned
- updates other unit tests to reflect changes
Merge pull request #4249 from PaulAdamDavis/fix-signin-styles

Refactor auth form styles
Change keyboard shortcut context

closes #4247
- added scope to every shortcut, the scope of shortcut is default to
  'default'
- add shortcut `enter` to modal which confirm the modal
- shortcut `enter` and `esc` now have scope 'modal'
- when lunch a modal, scope is switching to 'modal', and set back to
  'default' when close
- shortcut `enter` now confirm the dialog without conflicting with route
  shortcuts
Merge pull request #4265 from RaoHai/issue-4247

Change keyboard shortcut context
Merge pull request #4242 from cobbspur/meta

Adds Open Graph tags to ghost_head