Add limit=all consistently to users, posts & tags

refs #623, #4348

- this fixes sitemaps to list all posts, pages, tags and users
- makes the API behave consistently across all paginated resources
adds code injection admin frontend implementation, handlebar helpers + settings

escaping handlebars
Merge pull request #4573 from ErisDS/limit-all-sitemaps

Add limit=all consistently to users, posts & tags
Add XSL for Sitemaps

fixes #4555

- There's no easy way to declare an XSL with the node xml module, so I
  needed to move the declarations to both be strings
- Ideally the code to serve the XSL would also be inside the sitemap
  module, but I think we need to refactor a bit to get there easily
- Added the XSL from #4559, with minor amends to make the tables and urls
  display correctly
Merge pull request #4562 from ErisDS/issue-4555

Add XSL for Sitemaps
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
Merge branch 'master' of https://github.com/TryGhost/Ghost

Conflicts:
	core/client/templates/settings.hbs