Swap nav item & logo style order

Closes #4532

- Swaps the order of nav-item and .ghost-logo styles, as .ghost-logo was being overwritten by .nav-item, allowing a hover state
Add Labs, Nav & Grab icons

References #4535, #4534

- Add an Atom icon for Labs page
- Adds Compass and Grab icons for the Navigation page
Add Ghost version to config object.

No Issue
- Use Ghost version value that is already loaded instead of
  reading package.json from the filesystem and parsing it on
  every call into the configuration API.
Miscellaneous client cleanup

No issue
- Consolidate extension of Ember.Router.
- Remove unneeded local jshint flags and '_' as a global.
- Cleanup client README.md.
- Fix error message.
Merge pull request #4536 from PaulAdamDavis/labs-icon

Add Labs, Nav & Grab icons
Merge pull request #4533 from PaulAdamDavis/logo-no-hover-bg

Swap nav item & logo style order
Merge pull request #4538 from jaswilli/config

Add Ghost version to config object.
Merge pull request #4542 from jaswilli/misc-cleanup

Miscellaneous client cleanup
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.