Add validation to invite new user form.

Closes #3246
- Add a UserValidator to the validation engine that runs a set
  of validations based on the user status.
- Added validations for invited users and active users.
Re-add back to Users button and make whole user item a link.
Check the end of notification fade-out animation
Use bookshelf's model registry plugin

Refs #2170

This removes the circular dependency problem from our models thanks to
https://github.com/tgriesser/bookshelf/issues/181
- add the registry plugin
- switch all models and collections to be registered
- switch relationships to be defined using a string, which calls from the registry
Merge pull request #3247 from PaulAdamDavis/user-link-links

Make user list links and add "< Users" button
Merge pull request #3248 from jaswilli/issue-3246

Add validation to invite new user form.
Merge pull request #3256 from PaulAdamDavis/fix-notification-flicker

Check the end of notification fade-out animation
Merge pull request #3258 from ErisDS/model-registry

Use bookshelf's model registry plugin
Fire NProgress on User, Post, and Settings save

Closes #3037
- Created `NProgressSaveMixin`, which extends the `save` method of a model
  to fire NProgress.
- Extended `UserModel`, `PostModel`, and `SettingModel` with the new
  mixin.
- NProgress can be disabled by passing an options hash to the save function with the `{disableNProgress:true}`
- Now that the ValidationEngine isn't the only thing playing with options inside of `model#save`, refactored it to pass the options down the super chain.
Some sweets for my sweets
Merge pull request #3263 from joshk/patch-1

Some sweets for my sweets
sugar for my sugar
Merge pull request #3265 from joshk/patch-2

sugar for my sugar
Add status invited-pending for users

no issue
- added status invited-pending for invited users where sending the
invitation email failed
- removed console.log() from authentication.js
Add bower to travis cache
Merge pull request #3212 from novaugust/nprogress-save

Fire NProgress on User, Post, and Settings save
Merge pull request #3260 from sebgie/invited-pending

Add status invited-pending for users
Update Ghost-UI version to ~0.8.0

No issue

- Changes Ghost-UI version to look for any version from 0.8.0 onwards.
Merge pull request #3267 from ErisDS/cache-bower

Add bower to travis cache
Merge pull request #3268 from PaulAdamDavis/bower-ghost-ui-080

Update Ghost-UI version to ~0.8.0