Auto-save post at 60 second intervals

Refs #4259
- Auto-save the post at least once in any 60 second interval where
  there have been changes.
Enable :author as permalink option

Closes #3858
- urlPathForPost is now aware of the author’s slug, allowing for the
permalink setting to use :author
- Frontend controller only renders posts that have the specified
author, if one is given
- Extensive tests included
Swap 2 box shadows for 1 radial gradient

Closes #4340

Fixes a UI glitch where the vertical parts of a box shadow would show.
This fix changes those box shadows for 1 radial gradient, where only 5px of it is shown. No more vertical shadows.
Fix argument handling in gh-path helper.

No Issue
- Make helper behave as stated in description when variable
  number of arguments are passed in.
- Handle leading and trailing slashes when creating output
  url partial.
Merge pull request #4324 from jaswilli/timed-save

Auto-save post at 60 second intervals
Go between post list / post content with keyboard

Closes #3940
- Pushing left/right changes the "focus" of the posts screen to the list / content, respectively
- Once one of these has been focused, up/down keyboards work on that section in particular
- By default, the post list is selected
- Using the keyboard to focus one of the two makes it go "poof" to let you know you changed stuff
Merge pull request #4343 from jaswilli/gh-paths

Fix argument handling in gh-path helper.
Merge pull request #4341 from PaulAdamDavis/fix-floating-scroll-shadows

Fix floating header vertical shadows glitch
Use gh-path for nav bar dfallback avatar

Closes #4344

- Uses the `{{gh-path}}` helper to make the fellback avatar src work when Ghost is in a sub-directory
Merge pull request #4345 from PaulAdamDavis/navbar-image-path

Use gh-path for nav bar fallback avatar
Remove duplicate posts and tags on import

Issue #806

- Modify sanitize to check for duplicate posts and tags
- Update posts_tags when referencing removed duplicate tags
- Return both data and problems from importData
- Add tests for duplicate posts and tags with fixtures
Merge pull request #4338 from felixrieseberg/iss3858

Enable :author as permalink option
Merge pull request #4285 from novaugust/read-draft-shortcuts

Go between post list / post content with keyboard
Created image helper with absolute url option

closes #4231

- Adds {{image}} helper
- Adds image_spec test unit
- Updated {{ghost_head}} to use image helper
Consolidate SimpleAuth.AuthenticatedRouteMixin into AuthenticatedRoute

No issue
Peg node-sass dependency to 1.0.3.

No Issue.
- grunt-sass has node-sass as a dependency (^1.0.0) but the build
  requirements of node-sass version 1.1.0 have changed and will
  fail without a newer version of the gnu compilers.
Merge pull request #4355 from jaswilli/deps-sass

Peg node-sass dependency to 1.0.3.
Added screenshot to readme
Merge pull request #4351 from novaugust/authenticated-route

Consolidate SimpleAuth.AuthenticatedRouteMixin into AuthenticatedRoute
Redirect unauthorized users to signin from debug

Closes #4357
- Adds a call to `this._super()` in DebugRoute#beforeModel to hit the ember simpleauth mixin's beforemodel hook