adds code injection admin frontend implementation, handlebar helpers + settings

escaping handlebars
Code Injection - adds perms, shortcuts, icon, flag

refs #1993

- adds ctrl/cmd+s for save
- adds config flag
- adds icon on settings page, puts items in the right order
- sorts out permissions for all settings pages with consistent configuration
Adds markdown highlight support

closes #4574

- adds highlight showdown extension with tests
Merge pull request #4575 from ErisDS/highlight

Adds markdown highlight support
Merge pull request #4578 from ErisDS/pr-3489

Code Injection PR 3489 rebased and updated
Revert "Allow notifications to sit over the nav bar"
Match labels with element ids.

Refs #4578
- Match label "for" attributes with ids from the inputs they're
  labeling.
- Remove extra promise generation from ghost header and footer helpers.
Strip footnotes from excerpts

fixes #4572

- Remove both inline and bottom footnotes from excerpt output before stripping the remaining HTML
- No more red errors, black text or bold links in codemirror, as codemirror gets confused by footnote syntax. This is a step towards the new editor which has no syntax highlighting in the editor
Upgrading Casper to 1.1.3
Merge pull request #4581 from TryGhost/revert-4544-notification-over-nav

Revert "Allow notifications to sit over the nav bar"
Merge pull request #4582 from jaswilli/ci-adjust

Match labels with element ids.
Merge pull request #4580 from ErisDS/footnote-excerpt

Strip footnotes from excerpts
Fix settings header to allow dropdown menus to work