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
Created image helper with absolute url option

closes #4231

- Adds {{image}} helper
- Adds image_spec test unit
- Updated {{ghost_head}} to use image helper
Editor must be focused before allowing certain keyboard shortcuts.

closes #4253
- checked for the CodeMirror-focused class on the CodeMirror div
- used length to determine whether CodeMirror-focused has been added
- determines whether the editor has been focused on
Update Sass Grunt task output style

No issue

A recent update to NodeSass changed the keyword for output style from `style` to `outputStyle`.
This updates that, thus making the minfied CSS, actually minfied.
Merge pull request #4380 from PaulAdamDavis/sass-compress

Update Sass Grunt task output style
Merge pull request #4155 from jgable/importDuplicates

Remove duplicate posts and tags on import
Merge pull request #4317 from cobbspur/image

Created image helper with absolute url option
Merge pull request #4375 from DaveRobson/#4253

Editor must be focused before allowing certain keyboard shortcuts.
Save meta title and meta desc on post save

Closes #4369
- Renamed `editor-route-base` -> `editor-base-route` to bring it in alignment with the view and controller mixins
- Consolidated editor route code into editor-base-route
- Removed `serialize` from EditorEditRoute, as it reimplemented the ember default
Use nested output style for Sass

References #4389

As a result of https://github.com/TryGhost/Ghost/pull/4389#issuecomment-61637323 the best way to keep everything working nicely is to use the `nested` outputStyle for now.
Create Tags Management Stubs for Ember

closes #4260

- Adds "tags" route inside of the settings routes.
- Adds this route to router.js
- Links the route from the settings.hbs template
- Adds demo tags html in tag.hbs template
- Adds flag for tagsUI
Merge pull request #4385 from novaugust/save-meta

Save meta title and meta desc on post save
Merge pull request #4273 from cobbspur/tags

Create Tags Management Stubs for Ember
Merge pull request #4393 from PaulAdamDavis/sass-nested

Use nested output style for Sass
Update to Ember 1.8.1.

http://emberjs.com/blog/2014/11/04/ember-1-8-1-released.html
Merge pull request #4399 from rwjblue/update-to-1-8-1

Update to Ember 1.8.1.
Correct logError to log err which has undefined `message`

closes #4384
 - if err is `object` && `err.message` isNOT `string`, let err = `An unknown error occurred.`
Merge pull request #4384 from meowtec/fix-error-log

fix bug in which `logError` could not resolve an err without `message`