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.
Escapes strings

closes #4328

- Escapes metaTitle, metaDescription, blog url, blog title, tags
- Updates tests
- Adds test for metaTitle and metaDescription with double quotes
Fix up title/slug handling on posts.

Refs #4323
- Remove any titleScratch observers that may be hanging
  around on the post-settings-menu controller from previous posts.
- Change logic around when to regenerate slugs on posts that are
  "(Untitled)" so that slug generation continues after slug no
  longer resembles "untitled."
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 #4332 from cobbspur/escape

Escape meta title and description
Merge pull request #4337 from jaswilli/title

Fix up title/slug handling on posts.
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
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
Merge remote-tracking branch 'upstream/master'

Conflicts:
	core/client/assets/sass/layouts/content.scss