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
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
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