New Post UX behaviour.

refs #1351
- Make generateSlug a static method on base model
- Change client behaviour (unfinished)
- Add new getSlug API method
Extending halfdan’s work for UX settings improvements

#1351

- prevent a new post (not saved on server) from
updating its slug/date to the server

- fix jshint

- add back creation of a posts slug upon a post creation

- update for rebasing

- hide ability to ‘delete this post’ from post settings
menu when a post hasn’t yet been saved to the server
Add update notifications

closes #1464

- adds opt-out via updateCheck:false in config.js
- update check is done on admin index, but doesn't interfere with rendering
- adds update check module, which gets the usage data, makes the request and handles the response
- adds two new settings to default-settings, one for next check time, and one for whether to show the notification
- adds a new rejectError method to errorHandling
- adds a new helper for displaying the notification

Conflicts:
	core/server/helpers/index.js
	core/test/unit/server_helpers_index_spec.js
Merge pull request #1832 from ErisDS/issue-1464

Add update notifications
Merge pull request #1839 from hswolff/1351-post-settings-ux

New Post UX behaviour.